I'm here to help you with the customer that wasn't able to pay through your emailed invoice. I'd like to know if your customer encountered any errors during the process and let me know in the comment section.
For now, I'd suggest advising your customer to perform the browser troubleshooting steps. The data issue in the browser happens when there are updates on the browsers or the online program and needs to be refreshed.
Begin with using an incognito browser when logging into their email to pay. This is done to eliminate any unwanted data issues in their browser. If they're able to pay, you can also let them go back to the regular browser and clear the cache to avoid getting the same issue. An alternative way is to use another supported browser.
Hold down Command + Shift + N for Safari.
Press Ctrl + Shift + P for Microsoft Edge or Firefox.
Press Ctrl + Shift + N for Chrome.
If they're still not able to pay it from there, you can let them use ACH method. Then, manually record the invoice payment and send it to your customer. To know more about this process, please check this article: Record invoice payments in QuickBooks Online.