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Thanks for posting here in the Community, @bmisouthernpride.
Most common display issues might be caused by too much cache files in the browser. This is the possible reason your customers are unable to see your invoice details.
To resolve this, we can let your customers open the invoice in a private or incognito window. This type of window doesn’t store any cache that causes a browser to load slowly.
If it works, then they can go back to their regular browser and clear the cache storage to fix the issue. If it won't, have them use another supported browser.
To learn more on how to receive and categorize invoice payments in QuickBooks Online, read this article: Record invoice payments in QuickBooks Online.
Please let me know how it goes. I'll be looking forward to hearing back from you. Have a great day!