OK, so I paid...

[Wuxiapian]

... for 1st class recorded postage and seller has sent item 2nd class recorded.

Do I mention it when leaving feedback or let it go?

Much thanks.

Only if there is a big price difference in the P&P. Personally, I wouldn't bother. It's not worth the potential agro.

Don't communicate with feedback. If you have a problem always contact the seller first. In this case contact them pointing out their error; they should refund you the difference as you paid for a service you didn't receive.
IF they ignor you, don't refund, get arsey etc THEN you can mention it in feedback