After sending the checkout to the adopter's phone, you will see a notification once the adopter has completed both the contract and the payment. You can also manually check the status by hovering over the shopping cart icon in the top navigation bar and clicking "Shopping Cart Manager." The action link will say "Resend Checkout" until the transaction is complete.
At that point, the action link will say "Send Documents" which triggers a pop-up window to print and/or email the receipt, adoption contract, medical paperwork, etc.
Signed adoption contracts can be found on either the animal's record or the adopter's record in the documents section, which can be accessed by clicking the paper clip icon.