These are the Display and Packaging Companies that I use in the UK.
Mounting your artwork makes them so much more presentable. I buy my mounts in UK from Cotswold Mounts Their mounts are an excellent quality, they have a great range of sizes and colours including presentation bags and their service is very quick and efficient.
I personally choose to use white core mounts and I choose Tint 1 as my mount colour.
I buy cardboard envelopes for posting my art. I find these are the best method of sending my mounted art as only unmounted art can be posted in a tube. The Company that I use in UK are Ideal Envelopes and their service is also very quick and efficient.