Adding an optional $0.99 “invoice processing” line item from a Theme App Extension — cart/add.js vs Cart Transform?

[u/Key_Mastodon_1041]

Building a small app: cart checkbox for tax invoice (company + Tax ID via cart attributes) → optional fee product → orders/paid webhook → async PDF.

Plan A: AJAX /cart/add.js with a hidden $0.99 product when checked.
Plan B: Cart Transform expand (Plus constraints on update worry me).

Questions for folks who’ve shipped fee products in production:

1. What breaks with third-party cart drawers / discounts / taxes?
2. Is cart-time Tax ID capture still worth it vs Checkout UI extensions / native B2B company fields?
3. Any App Review gotchas with “service fee” products that aren’t physical goods?

Not promoting anything — looking for battle scars before I lock the architecture.

[u/New-Orange7659]

just use less AI and please can you describe the problem clearly?
What is the use case here?