There's a thread on SocialHub where I posted some ideas about payment links: https://socialhub.activitypub.rocks/t/fep-8c3f-web-monetization/2719/21
I think payment information should be represented as objects with type Link (or it could be a new subtype of Link such as PaymentLink), because in most cases you can generate payment URI of some kind. For example, here's how we can represent LN payment link:
Can be an attachment or a tag, IMO both are fine. Subscriptions links in Mitra are currently added to attachment array on actor object.
I've been planning to propose it as a FEP, so it's good to see people requesting such features.