Ok, this may be dumb, but it seems to me like listings added via a completely different app should not default to using Cashu via DM unless the person listing it manually goes to Shopstr and changes it over to use their Lightning address. It should probably default to the payment method they have set up in their kind 0.
Discussion
That's a good point since most might not be familiar with Cashu; defaulting to the Lightning address then Cashu if no Lightning address exists is probably a better way of handling it.
Ideally, there will be also be a NIP-89 handler that recommends the merchants' preferred app, and the client would link out to that app to finalize payment and/or marketplace implementers add support for the 'payment_preference' field in kind 0 (this is specced out in the [revised marketplace NIP](https://github.com/GammaMarkets/market-spec)).