Welcome to the Community. I'd be glad to offer some info on a Pay Now button for statements.
As you mentioned, the Pay Now button shown on invoices is a handy way for your customers to pay, so you can get your money even faster. At this time statements are used to show customers their account balance. Statements display a summary but customers aren't able to pay through this type of document. I'll be sure to pass your feedback along to the Product Development Team to let them know you'd like to see a Pay Now link on statements.
I would have to agree that having a Pay Now button on the statements is convenient for you and your customers.
While we don't have these options, I will take note of your experience and forward this to our product developers. Our engineers make improvements in QuickBooks Online by listening to our customers' suggestions and comments.
I'll also give you an update here as soon as this feature is implemented.
As always, you can check regularly for product improvements and feature enhancement through this link: QuickBooks Blog.
That should get you on the right track. If you have other questions, I would be happy to answer them. Just post it here as a comment and I'll take a look at it. Enjoy your week.