For tyres you're better getting the dimensions and approx weight (stack them in two's maybe) and going to interparcel, parcelmonkey or the one above and getting a quote yourself.
Usually they're happy to accept them in strong black bags with tyres taped together, although a wrapping of bubblewrap/cardboard would be nice at least.
The likes of PF are great for interiors and alloy/tyre combo's, but pretty sure I got two 17" tyres picked up and delivered next day for £9.50ish early in the year.