Hi Lucy-Wieser,
Being able to have customers pay directly from the invoice is what the payments option is all about. I can help you get this working.
QuickBooks Online Payments makes receiving payments from your customers easy with the pay now option. If the customer isn't seeing the pay now option it could be due to a setting in your clients QuickBooks Online account being turned off. To make sure the pay now option shows on the email you can go to Gear > Account and settings > Sales > Online Delivery and make sure that the drop down menu's not set as plain text as this will stop it from showing. If that's set up correctly and the customer still can't see it have your client send the email to themselves and see if they can see it. If they can then it may be on the customers side which they can try clearing their cache and cookies from their internet browser and trying to view it again or try a different internet browser all together. You can provide them this article on how to clear cache and cookies for different internet browsers.
Hope this helps.
