Selling on POS and using Wallee at the same time is not a neat experience.
You have to enter your Order Details /Amount manually into the Terminal and you can also not use the advantage of selling a POS Order with an Invoice for the Customer. At least not with big manual effort.
Thus it would be nice to have a small POS App integrated into Shopify POS running on Order Completion (see here https://www.shopify.com/partners/blog/point-of-sale-apps)
The "Wallee POS connector" should have the Option to configure the Terminal ID and enter an API User.
It should do nothing other than send the order information to the configured Terminal over the Terminal API. On your Terminal you can then finish the transaction on the Terminal by paying with your choosen card.
The Transaction in Wallee is automatically created and the Merchant Reference Matches the Shopify reference for easier detach and find later.
Additionally it would be nice to be able to also choose to send the transaction to your Invoice Connector so that the transaction could also be paid with an Invoice within wallee instead over the terminal.
So basically the app would do nothing other than fetch and send transaction data of a Shopify order finished manually in POS App to Wallee or one of its Terminals.