I can't tell which seller uses which postal service. What I do know, is, as the parent of three busy children/teenagers and then young adults, plus running the A.V. services of a busy school, I was handling parcels of all shapes, sizes and weights on a near-daily basis for the better part of two decades. This included fencing gear, musical instruments, computers, tools for me, machines for me, satelite equipment, you name it. I came to the unshakeable conclusion that Royal Mail was maligned regularly and perhaps unfairly, given that it delivered the vast majority of its items, maybe not exactly on time, but pretty close.
I also concluded that there were only two types of courier: the professional ones who said what they meant on the tin, who had been around delivery services for a while, or were experienced in the trade before they ventured into it as an independent entity, and did it properly. Then there were the other come-lately shysters, buoyed up by venture capital, who spouted business acronyms and slogans, who were not interested in customers; when they hit a problem, charged extra for something they should have foreseen in the first place, had grandiose 'tracking' systems that frequently were 24-36 hours out of date and showed items "still at hub" when I had unpacked them.
C.P.C. (Farnell), Screwfix and Axminster etc etc have free delivery country-wide above a certain limit and deliver on time. So, by-and-large, does Royal Mail. There are 16 flights a day into our two airports. There are several million tons of shipping into our ports and at eight-twelve freight/passenger ferries per day on top of that. Don't give me that guff that we can't be delivered to, or that a £7 tin of paint requires a £15 'surcharge' because I live across twenty three miles of water....or, put another way, less than the distance to the Isle of Man......SO, WHY DO I NEVER SEE: "Cannot deliver to the Isle of Man"???
Utterly lacking in gruntle, off the piste, fed up, Sam