Express prepaid C5 envelopes are $72.65 per 10 pack. About $4.80 per envelope more than standard prepaid C5.

Express prepaid B4 envelopes are $86.30 per 10 pack. About $4.40 more than standard prepaid C4.

If the OP chooses to send 'free' by standard post, then adds the entire cost of express if that is chosen smacks of postage piracy. And if the entire cost of express is in the realm of $12 they are not quoting for letters but for parcels. Which start at $8.95.

I can understand them ratchetting up express costs to avoid people using it, while still retaining the 'advantages' of Plus, but it also behoves the seller to keep up with eBay policies in this regard.