Building agents that can purchase things — what is your setup?

Experimenting with Claude-based agents that handle procurement tasks (SaaS renewals, API credits, small B2B purchases). Curious what setups others are using.

Current pain points:

The checkout problem. Most merchants don't have APIs. Getting Claude to navigate a checkout form is fragile — it breaks on CAPTCHAs, 2FA, and any UI change. Has anyone solved this cleanly or are you restricted to vendors with actual purchase APIs?

The authorization problem. How do you tell Claude "you're allowed to spend up to $200/month on developer tools but need approval for anything else"? Right now I'm doing this in the system prompt but that feels hacky.

The audit problem. Claude's tool use logs don't naturally produce compliance-ready audit trails. What's your logging setup for financial actions specifically?

Also curious: has anyone built against UCP (Universal Checkout Protocol) yet? Shopify and Google are pushing it but I haven't seen real implementations discussed much here.