You don't have to. You can pay anytime by clicking the "Buy Now" button at the top right in black and white.
TurboTax Online charges you per return based on your tax situation. For Example, if your return includes self employment income from a business you own you will be charged for TurboTax Home & Business when you file. On the other hand, if you select Home and Business at the beginning but you return ends up qualifying for TurboTax Free you could have no charge at all. I would wait to the end to pay.