So, the reason I would want callbacks, is that I can generate all these lnurls per-pubkey given that the client performs the auth. I may still have to generate just as many (or i may be able to re-use them). But then, my backend does not know immediately that it was paid, the polling cycle has to check all the invoices and the less quick it is, the more likely the user will continue to receive the notify (and be blocked from access). This could be on the order of minutes. With a callback, the payment check can happen and we don't need to rely on polling. (polling bad, callbacks good, that's compsci)