Only final item added to cart is charged to Square

Problem/Motivation

If someone adds more than a single item to the cart, then the final item added to the cart is what actually gets charged to Square.

Variable name re-use is what is biting us. We have the same variable $square_amount reused throughout the payment gateway.

Proposed resolution

Fix the bug. Super quick. Let's separate out our variable names and name the $square_amount to $square_total_amount when we are dealing with the square total amount.

Remaining tasks

Review patch.

User interface changes

API changes

Data model changes