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.