Adding additional 2Checkout fields for a better customer experience.

The `ship_name` field is currently missing so the buyer must enter it on 2Checkout's hosted page.

If all billing information (and shipping information if applicable) is passed the `purchase_step` parameter can be passed with the name of the checkout step to display to the buyer when they arrive on 2Checkout's hosted page.
Possible values are ‘review-cart’, ‘shipping-information’, ‘shipping-method’, ‘billing-information’ and ‘payment-method’.
https://www.2checkout.com/documentation/checkout/parameter-sets/pass-through-products

2Checkout referral partners can optionally pass the `2co_cart_type` parameter to identify themselves or their cart software.