The invoice might been edited after sending it to the customer, @LS1755.
Let's go back to the invoice transaction that you've created to your customer and resend it. Once done, inform him/her to make a payment for it.
If the customer is still getting the error, delete and recreate the invoice. Then, send it back and inform him/her to click the link on the latest invoice.
You can let your customers try these basic troubleshooting steps. First, use private browser when paying the invoice.
If it works, they can go back to their original browser and clear the cache. It might be that their cache is full which cause this unusual behavior. They can also use another compatible browser with QBO as an alternative solution.
I'll be around if you have other concerns.