Large pay now button on the first page. But it is of dubious use unless you forgot your wallet. You won't get your free coffee that way. They want you to use the pre-order feature and avoid the "time consuming" process of ordering via the cashier. But if you are used to idling in a drive-thru line, saving time probably isn't your priority, so this app might not be for you.
Starbucks only has 10% of their customers use the pre-order feature. Many American chains have launched mobile order in the USA this year, but I'm not that sure it will catch on. Kind of like the self tellers at the grocery stores. Sometimes useful, but not always. Still, I remember elevator operators and gas station attendants, so you never know.
It might take a company to go fully digital like LaPresse, getting rid of their paper edition. Their tablet readership is much higher than for the Toronto Star that has both. Starbucks is trying a mobile order only store in a Seattle building, so it will be interesting if that spreads. But this might be a solution in search of a problem.
That being said, there is no joy equal to walking past 15 people to pick-up a hot coffee. Even better if you pre-ordered food and it is lunch time.