"To finalize the transaction" means completing or finishing the process of buying or selling something. It means all the necessary steps have been taken to complete the deal and make it official. Full definition
It's where all the parties get together to finalize the transaction by exchanging signatures, checks — and ultimately, house keys. (usbank.com)
It seems you can call and start the award booking process and wait to finalize the transaction until days before flight date, if that's how you roll. (viewfromthewing.boardingarea.com)
Make sure to review the amount of BTC you're spending and the amount of RVN you're buying before finalizing the transaction. (finder.com)