Thanks for dropping by the Community, @susan.oehler.
To ensure that I'm on the same page, may I know the specific error message your client is getting when making a payment online? This will help me provide the best resolution for the issue.
In the meantime, have the client log into their email account using a private browser (incognito). There are times that the browser is full of frequently accessed page resources, causing some unusual responses.
- Press Ctrl + Shift + N (Google Chrome).
- Ctrl + Shift + P (Firefox or Internet Explorer).
- Control + Option + P (Safari).
If it works, they can return to their default browser and perform a clear cache. Every so often the cache becomes overwhelmed with older data which can potentially cause viewing and performance issues. Clearing it will refresh the system. However, if the issue persists, they can use other supported browsers.
Just in case, I'll add this article for future reference: How to let customers pay their invoices online.
Please let me know if you have any other issues or concerns by clicking the Reply button below. I'm more than willing to assist. Enjoy the rest of the day!