I don't think that needs to be solved tbh, at least not at first, it just might be that this is solved via generic npub (both customer and service provider) reputation

the service provider doesn't need to prove anything, because that'd be too costly at scale, instead, the incentive is aligned in that the npubs might want to transact again in the future.

i.e. if I'm a person buying a midjourney image, the image is great and I don't pay the service provider, I risk that that service provider (and others watching my behavior) will not serve me in the future

there is risk to the service provider, but that's up to the market/service providers to price/mitigate

the risk could be pushed to the user (e.g. via hodl invoices) but service providers are better positioned/have more incentives to mitigate risk than end users