package hasn't arrived...

[balia]

I have the proof of posting, dated 26th of last month, and the buyer has just messaged to say they haven't had it. Now what do I do?

ask them if they could check with their sorting office & neighbours, some posties don't leave a card

If after 15 working days, 3 weeks, it's not arrived you should refund then claim for lost item with Royal Mail. So I think wait until what the 16th of Nov, them take it from there.