There are certainly times when it is appropriate to pay for things at the pharmacy counter, like when a customer has a questions about an over-the-counter drug and possible interactions with another drug they might be taking. However, I agree with many below that a limit would be a good solution.

However, the situation in the letter might still have occurred with this "limit" policy in place. Their would have been problems ringing up 10 items with would still take a lot of time to correct.