In case the integrated payment form shows a page not found error this normally relates to the setup of the Proxy URLS of the Sales Channel. By default the application sets up Proxy URLs. It is possible to edit the proxy URLs, however it is not recommended.
In case you want to change the URLs of the integrated payment form or in case you get an 404 error page instead of the payment form please proceed as follows:
Make sure the Proxy URLs that you setup in Shopify under Settings > Sales Channels > View Details are exactly identical to what is setup in wallee under Shopify > App Proxy Path.