Add Zinc to Payments, Checkout & Tax

[Illyism]

Summary

Add Zinc to Payments, Checkout & Tax, alphabetically after Visa Intelligent Commerce.

This is the official Zinc API agent skill (zinc.com), not the chemical ZINC compound database.

• Docs: https://www.zinc.com/docs/v2/agent-skills/overview
• Install: npx skills add zincio/skills --skill universal-checkout
• What it is: Agent skill and API for discovering, buying, tracking, and returning products across Amazon, Walmart, Target, and 50+ US retailers.

Zinc belongs next to Rye as a universal checkout API that lets AI agents purchase from major US retailers.

Test plan

• Confirm the new entry is alphabetically ordered in Payments, Checkout & Tax
• Confirm the description matches the contributing format (- [Name](url) - Short description ending with a period.)
• Confirm the link is the canonical skills repo, not an affiliate URL
• Confirm this is not a duplicate of an existing entry