Error occurred while purchasing a label. Please try with a different delivery method

This question seems to have been raised several times, but I cannot seem to find a definitive answer.

We are unable to buy Royal Mail Tracked 24 labels, which is a requirement for Prime.

The account is used outside of Amazon without any issue, but we just constantly get the "Error occurred while purchasing a label. Please try with a different delivery method" message when trying to purchase through Buy Shipping. Non-Prime orders can have their labels printed outwith Amazon, and tracking details uploaded. And we can buy Tracked 48 without issue within Amazon, but not the Tracked 24 that we need.

Does anyone have any ideas on how to fix, as Amazon help haven't sorted?