Controls how the update is applied:

• PATCH - Only update specified fields, retaining existing values for unspecified fields
• REPLACE - Replace only the specified fields at the top level of the payment link object. Unspecified top-level fields (like options, state) are preserved. When updating nested objects like options or state, the entire nested object is replaced with the provided values. (default)

Example behavior:

1. Original payment link has: {expires: "2026-08-01T00:00:00Z", options: {currency: "USD", amount: "10.00"}}
2. Update with PATCH mode and {expires: "2026-09-01T00:00:00Z"}: Results in {expires: "2026-09-01T00:00:00Z", options: {currency: "USD", amount: "10.00"}}
3. Update with REPLACE mode and {expires: "2026-09-01T00:00:00Z"}: Results in {expires: "2026-09-01T00:00:00Z", options: {currency: "USD", amount: "10.00"}}
4. Update with REPLACE mode and {options: {currency: "EUR"}}: Results in {expires: "2026-08-01T00:00:00Z", options: {currency: "EUR"}} (note that amount is removed)