To clarify good advice received, over-the-counter Royal Mail T48 Large Letter costs £3.30 for tracked service with £75 insurance. Simple Delivery is cheaper and better with T48 including 10x the insurance value, plus protection from eBay for lost/damaged items and negative/neutral feedback.
And yes, you can enable Custom Postage to send RM 2nd Class Large Letter 250g for £1.90
This only has £20 insurance, but your proof-of-posting has a Reference Number on it which works as a crude tracking number, so it is good for smaller value items as you say. Good luck!