This is the full public function in /wp-content/plugins/woocommerce-amazon-ebay-integration/woocommerce-amazon-ebay-integration.php

	/**
	* proxy is used to translate local requests to the wordpress instance that represent
	* requests for UI and proxies those requests from the server back to Codisto
	*
	*/
	public function proxy() {
		global $wp;

		error_reporting( E_ERROR | E_PARSE );
		set_time_limit( 0 );

		@ini_set( 'zlib.output_compression', 'Off' );
		@ini_set( 'output_buffering', 'Off' );
		@ini_set( 'output_handler', '' );

		while( ob_get_level() > 1 ) {
			@ob_end_clean();
		}
		if ( ob_get_level() > 0 ) {
			@ob_clean();
		}

		if ( isset( $_GET['productid'] ) ) {
			wp_redirect( admin_url( 'post.php?post='.urlencode( wp_unslash( $_GET['productid'] ) ).'&action=edit#codisto_product_data' ) );
			exit;
		}

		$HostKey = get_option( 'codisto_key' );

		if ( ! function_exists( 'getallheaders' ) ) {
			 function getallheaders() {
				$headers = array();
				foreach ( $_SERVER as $name => $value ) {
					if ( substr( $name, 0, 5 ) == 'HTTP_' ) {
						$headers[str_replace( ' ', '-', ucwords( strtolower( str_replace( '_', ' ', substr( $name, 5 ) ) ) ) )] = $value;
					} elseif ( $name == 'CONTENT_TYPE' ) {
						$headers['Content-Type'] = $value;
					} elseif ( $name == 'CONTENT_LENGTH' ) {
						$headers['Content-Length'] = $value;
					}
				}
				return $headers;
			 }
		}

		$querystring = preg_replace( '/q=[^&]*&/', '', $_SERVER['QUERY_STRING'] );
		$path = $wp->query_vars['codisto-proxy-route'] . ( preg_match( '/\/(?:\\?|$)/', $_SERVER['REQUEST_URI'] ) ? '/' : '' );

		$storeId = '0';
		$merchantid = get_option( 'codisto_merchantid' );

		if ( isset( $_GET['merchantid'] ) ) {
			$merchantid = (int)$_GET['merchantid'];
		} else {
			$storematch = array();

			if ( preg_match( '/^ebaytab\/(\d+)\/(\d+)(?:\/|$)/', $path, $storematch ) ) {
				$storeId = (int)$storematch[1];
				$merchantid = (int)$storematch[2];

				$path = preg_replace( '/(^ebaytab\/)(\d+\/?)(\d+\/?)/', '$1', $path );
			}
			if ( preg_match( '/^ebaytab\/(\d+)(?:\/|$)/', $path, $storematch ) ) {
				if ( isset( $storematch[2] ) ) {
					$merchantid = (int)$storematch[2];
				}

				$path = preg_replace( '/(^ebaytab\/)(\d+\/?)/', '$1', $path );
			}
		}

		if ( ! $merchantid ) {
			$this->sendHttpHeaders(
				'404 Not Found',
				array(
					'Content-Type' => 'text/html',
					'Cache-Control' => 'no-cache, no-store',
					'Expires' => 'Thu, 01 Jan 1970 00:00:00 GMT',
					'Pragma' => 'no-cache'
				)
			);
			?>
			<h1>Resource Not Found</h1>
			<?php
			exit();
		}

		$remoteUrl = 'https://ui.codisto.com/' . $merchantid . '/'. $path . ( $querystring ? '?'.$querystring : '' );

		$adminUrl = admin_url( 'codisto/ebaytab/'.$storeId.'/'.$merchantid.'/' );

		$requestHeaders = array(
							'X-Codisto-Cart' => 'woocommerce',
							'X-Codisto-Version' => CODISTOCONNECT_VERSION,
							'X-HostKey' => $HostKey,
							'X-Admin-Base-Url' => $adminUrl,
							'Accept-Encoding' => ''
						);

		$incomingHeaders = getallheaders();

		$headerfilter = array(
			'host',
			'connection',
			'accept-encoding'
		);
		if ( $_SERVER['X-LSCACHE'] == 'on' ) {
			$headerfilter[] = 'if-none-match';
		}
		foreach ( $incomingHeaders as $name => $value ) {
			if ( ! in_array( trim( strtolower( $name ) ), $headerfilter ) ) {
				$requestHeaders[$name] = $value;
			}
		}

		$httpOptions = array(
						'method' => $_SERVER['REQUEST_METHOD'],
						'headers' => $requestHeaders,
						'timeout' => 60,
						'httpversion' => '1.0',
						'decompress' => false,
						'redirection' => 0
					);

		$upload_dir = wp_upload_dir();

		if ( is_multisite() ) {
			$certPath = $upload_dir['basedir'].'/sites/'.get_current_blog_id().'/codisto.crt';
		} else {
			$certPath = $upload_dir['basedir'].'/codisto.crt';
		}

		if ( file_exists( $certPath ) ) {
			$httpOptions['sslcertificates'] = $certPath;
		}

		if ( strtolower( $httpOptions['method'] ) == 'post' ) {
			$httpOptions['body'] = file_get_contents( 'php://input' );
		}

		for ( $retry = 0; ; $retry++ ) {

			$response = wp_remote_request( $remoteUrl, $httpOptions );

			if ( is_wp_error( $response ) ) {
				if ( $retry >= 3 ) {
					$this->sendHttpHeaders(
						'500 Server Error',
						array(
							'Content-Type' => 'text/html',
							'Cache-Control' => 'no-cache, no-store',
							'Expires' => 'Thu, 01 Jan 1970 00:00:00 GMT',
							'Pragma' => 'no-cache'
						)
					);
					echo '<h1>Error processing request</h1> <p>'.htmlspecialchars( $response->get_error_message() ).'</p>';
					exit();
				}

				if ( $response->get_error_code() == 'http_request_failed' ) {
					$certResponse = wp_remote_get( 'http://ui.codisto.com/codisto.crt' );

					if ( ! is_wp_error( $certResponse ) ) {
						@file_put_contents( $certPath, $certResponse['body'] );
						$httpOptions['sslcertificates'] = $certPath;
						continue;
					}
				}

				sleep(2);
				continue;
			}

			break;
		}

		if ( defined( 'ADVANCEDCACHEPROBLEM' ) &&
			false == strpos( $_SERVER['REQUEST_URI'], 'wp-admin') ) {
			$_SERVER['REQUEST_URI'] = '/wp-admin'.$_SERVER['REQUEST_URI'];
		}

		status_header( wp_remote_retrieve_response_code( $response ) );

		$filterHeaders = array( 'server', 'content-length', 'transfer-encoding', 'date', 'connection', 'x-storeviewmap', 'content-encoding' );

		if ( function_exists( 'header_remove' ) ) {
			@header_remove( 'Last-Modified' );
			@header_remove( 'Pragma' );
			@header_remove( 'Cache-Control' );
			@header_remove( 'Expires' );
			@header_remove( 'Content-Encoding' );
		}

		foreach ( wp_remote_retrieve_headers( $response ) as $header => $value ) {

			if ( ! in_array( strtolower( $header ), $filterHeaders, true ) ) {
				if ( is_array( $value ) ) {
					header( $header.': '.$value[0], true );

					for ( $i = 1; $i < count( $value ); $i++ ) {
						header( $header.': '.$value[$i], false );
					}
				} else {
					header( $header.': '.$value, true );
				}
			}
		}

		file_put_contents( 'php://output', wp_remote_retrieve_body( $response ) );
		exit();
	}