Hey Bart,
You might not need to do anything custom at all.
Shopify lets you add a manual payment method to your store which customers can use to checkout with. I've attached an example I use in my development store which lets me skip the payment details.
If you setup a Manual Payment -> Custom Manual Payment then you can create a "Pay instore" option for customers. Here's Shopify's docs on how to set it up, it should take maybe 15 minutes.
https://help.shopify.com/manual/payment-settings/manual-payments
Eric Davis