With the exception of pastel paintings, I send all of my artwork loosely rolled in a reinforced postal tube. Pastel creations are sent sandwiched between four 5mm foam boards and covered in bubble wrap.
For UK customers I will always post using Royal Mail's Special Next Day Guaranteed service. For international destinations I use Royal Mail's 1st Class Tracked.
The customer is responsible for postage charges so if there is an alternative service that you'd like me to use, please mention it during the commission process.