Yes, I'd move on too. Just think, someone else probably had their phone turned off for non-payment, as they can't find any record that s/he paid, so they probably wouldn't credit their account properly, even if they showed the cash receipt.
Well whoever it was, apparently proved they made the payment, or US Cellular wouldn't have sent a letter telling us it was credited to our account by mistake. My guess is their phone got shut off before they realized what happened, and then they figured it out and got all over US Cellular about it.
Switch to Tracfone. It's ALL prepaid, You can choose how many minutes & plan you want each month. It's probably less than what you pay now, depending on how much you use your phone. AND they have auto-deduct for each month with a slight discount.
You can also "Bring Your Own Phone" and port your phone number over. So you can probably use the same phone & phone number you have now and only change your service provider.
I'll help you out if you are interested in going that route.
NOTE: If you want to keep your current phone number, your current phone
must be active with that number, so that you can port the number over to the new carrier. This is for ANY carrier you decide to switch to.