You've helped me see the source of the issue: my item is above £10.

I had no idea the item must be below £10 to qualify for Custom Postage. I don't understand why this is, because several items I plan to sell (PS1 games, some without the case) are worth above £10 and below £20, and cost £1-2 less than £2.70 to post.

I suppose I could not purchase the postage via eBay and do it directly from the post office instead, since I may have quite a few items in this price and postage range.

Thanks for your help, you've saved me trying to figure out the problem.