Shipping labels - Royal Mail tracking numbers

Hi,
I purchase all of my shipping labels through amazon, and send everything by Royal Mail.

If customers claim that an item was never received, the first thing I do is to go back into the order details, and click “reprint label”, as the label contains the tracking number number. I can then check this on the royal mail tracking website, as every now and again the ‘missing’ items have in fact been delivered!

However, the problem is that the “reprint label” option disappears after about 30 days, which makes it impossible to check on older orders. Is there any way around this problem? Is there some way to view shipping labels from further back than 30 days (it is possible on ebay), or to find the tracking numbers that were generated when the labels were purchased?

many thanks