I"m going to answer my own question.  Talked with Shopify Retail Support.  This functionality (finishing up online orders in store using custom payment) is not supported by Shopify POS (or POS Pro). I've found an App called Sales Terminal POS which W...

OK, I totally misunderstood the reply from HFR.I now understand that you can have custom payment as an option in the POS app.  I thought HFR was talking about custom payment in the online app.In any case, for me, when I try to use a custom payment ty...

THat's really disappointing.  I've been experiencing this issue and I thought it was my mistake.  Have you gotten any response?  or resolution?

I'm trying to collect payment for an online order using  the custom payment button on the POS app.  However, whenever I try it, I get the screen which says it will "Order will be marked as paid.  Custom payments are not processed by Shopify." and whe...

I have a similar question.  I have setup a Custom payment type.  When a user selects the payment type, in web (not in POS), the order is NOT marked as paid.  I wouldn't expect the order to be marked as paid as manual payment could be paid with Purcha...