It uses the nostr wallet connect mechanics to allow a service provider to request that a specific wallet pay a specific invoice. You can do that whenever you want, whether at checkout or once a day / week / whatever.

Then on the wallet side you can either do something like loading it on next open or even push notification to show the user there's a pending payment request. You can even allow the user to turn on auto pay w/ a budget if they wanted that too.

It's a protocol that would across any wallet and doesn't need the user to be online all the time, or any specific server even.