I have noticed that when completing a purchase and hitting the pay button, there is no obvious indicator that anything is happening. Sometimes there is a long delay between hitting the pay button and being redirected to the Thank You page. I feel like some customers might think it didn't work and might abandon the purchase.
I want to suggest once the pay button is clicked that you swap the button text to something that indicates processing of the payment. Maybe just the word "processing..." might work. Even better would be a loader animation of some sort.