My imaginary use case was a market stall selling vegetables, the owner wants his customers to pay with crypto but doesn't trust his sales people with access to his wallet. It would also work well for a bar, a shop, girl-scouts selling cookies, anywhere that sales are made face to face.
The salesperson uses it on their phone, which connects to a server (ie: a laptop) that the owner leaves running at home. It does simple order processing, checks for payments received & a few other things, but as it can ONLY receive funds the shop owner doesn't have to trust anyone.
Here's a summary of what it does :-
Here's a link to the prototype/development site which is running on mainnet :- (https://chilebob.duckdns.org)
The project has been running for a while, here's a link to screenshots, more information & discussion on the forum :- https://forum.zcashcommunity.com/t/zcash-point-of-sale-prototype/33019
I estimate it will take 30 days to finish, mainly because this is a hobby :-)