It's a dynamic market so fees can vary depending on congestion, BTC price, etc. Expected time to first block confirmation is around 10 minutes assuming your fee is high enough to be included in the next block (when blocks are full, miners prioritize transactions with higher fee).

When you send a transaction, you get to pick what fee you'd like to use. For this kind of service, transaction speed isn't really important so you can get away with setting a low fee.

Here's a 0.15$ fee transaction that was mined just 6 minutes ago: https://mempool.space/tx/2e66ba1758e233ff1b2395c7db55f8327de...

By "off-chain", I meant something like BuyMeACoffee letting users top up a wallet via an on-chain deposit, and then handling tips internally (e.g., in a SQL database). That way, small payments don't each require a blockchain transaction.