If small and not breakable, Collect+. Been fine so far.
If large and/or breakable, charge buyer more postage, use a proper courier, e.g. DHL.
I avoid myHerpes, they lost 30% of the parcels I sent with them.
I avoid any service that requires going to a post office, because they all appear to be staffed by unhelpful jobsworths who don’t want you there, cluttering up their nice stationary shop and pension collection point, getting in the way of their 10am chat with Gladys from up the road.
My favourite Post Office refusal was being told I’d written the address too large, so there was no room for the barcode sticker to go on the front. Lets not get into the physics of there being 5 other surfaces to put it on – it *had* to be put in the top corner on the address face. Jesus wept.