Add Cart Line Items

Adds line item to the *Cart*. **Usage Notes** To add a custom item use `custom_items`. Overriding a product’s `list_price` will make that item ineligible for V3 product level promotions. If a product has modifiers, omit the `variant_id` and instead use the `option_selections` array to describe both the **variant** and the **modifier** selections. Please note that this API endpoint is not concurrent safe, meaning multiple simultaneous requests could result in unexpected and inconsistent ...