Downloadable product quantity is handled incorrectly in Add To Cart API

[harshit-webkul]

Describe the bug
When adding a downloadable product to the cart using the Add To Cart API, the quantity is not processed correctly.
Instead of validating or restricting the quantity (typically downloadable products should not allow multiple quantities), the API behaves incorrectly—either allowing invalid quantities, giving wrong totals, or adding unexpected cart entries.

To Reproduce
Steps to reproduce the behavior:

1. Create or use a downloadable product.
2. Hit the Add To Cart API
3. Pass a payload with quantity > 1
4. Check the cart response and cart items.

Expected behavior

• Downloadable product quantity should:
  • Either be restricted to 1,
  • OR validated properly with a clear error message.
• Cart should not allow invalid quantities for downloadable items.
• Cart totals should calculate correctly based on allowed rules.

Screenshots
If applicable, add screenshots to help explain your problem.

[rohansingh754]

We have reviewed this behavior on the Bagisto core side.

In Bagisto, downloadable products do not provide a quantity input on the storefront, but the quantity is not explicitly restricted at the cart level. When the same downloadable product is added to the cart multiple times, the system increments the quantity instead of blocking it.

From the API perspective, this behavior is consistent:

• Adding a downloadable product multiple times via the API increases the quantity.
• Sending a single API request with a quantity > 1 results in the same outcome.
• Cart totals are calculated based on the resulting quantity.

Therefore, the Add To Cart API is working as intended and mirrors the core Bagisto cart logic. There is currently no validation in the core to restrict downloadable product quantity to 1.