Why not use Royal Mail still? £19.45 doesn't sound too bad. Especially if your fully insured with that! Sending £200 - £250 item insured up to just £100 seems a bit of a risk to me. When your only saving £3-4 doing so. Who's going to reimburse your buyer the full amount if an item gets lost?
If I was your customer, I'd prefer an item sent via Royal Mail rather than a courier. You can make sure its with your customer by 9am the next morning (meaning they won't have to wait in all day for it to arrive). Thus, leaving you with happy customers. If I missed the post, I could collect the item quite easily (as Royal Mail have sorting offices in the majority of towns). Or I could pay to have it sent to my local post office (is it 50p?). To do that with a courier would be in some cases very difficult.
These are a few things I would consider, rather than saving a few quid' think about your customers.
