SecurionPay
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`purchasee/3`
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* Implemented two `purchase` clauses:
- `CreditCard` struct (a `card_id` is NOT generated)
- `card_id` (`customer_id` must be provided in `opts`)
* Does not support optional arguments like shipping, billing etc.
* In both cases, the Gateway API is hit only ONCE.
Docs
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* Added docs for `purchase/3`.
Tests
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* Added integration tests for `purchase/3`, uing ExVCR.