If the postage is capped, you know the maximum when you list, and presumably know what it will cost to post, then you increase the start price.
You can't avoid the Cap. At the end of the day, it is what you receive, not the breakdown between price and P & P that matters.
Looking at your items, mostly using tracking is OTT, your proof of postage can be used to make a claim for items up to £20 if lost or damaged. Only over £20 is Signed For really essential.