The question of framing is, of course, very personal. My current preference is for a limed wood effect, 5 cms wide and 2 cms deep with a right angled outer edge. This, coupled with a wide off white mount, gives even the smallest of paintings some heft.
I do not have my own framing workshop and can therefore only offer this one style of frame. If you wish to have something different the simplest and cheapest option is to buy unframed. This also removes the risk of posting glass of course. An unframed painting package includes a mount, a sheet of acid free barrier paper and a backing board. The fully framed package will also include D rings and string for hanging.
An example showing the frame style and mount is shown.