If a shop can't repair it 100% then you took it to the wrong shop. This is a very simple repair for any reputable body shop to fix. With a heat gun and a wet rag the shop will be able to shrink the stretched plastic. They can then use a plastic based body filler to finish out the work. Shouldn't be anything more that 3-4hrs of body labor.
I have noticed the word Bondo being thrown around a little. Bondo is a type of body filler, and in my opinion it is one of the low end fillers as well -- Rage is tops when it comes to body filler. However, this is a plastic bumper so a body fill should never be used. Make sure the shop you take it to is familiar with a plastic filler called fusor. The bumper is a flexible part, fusor can flex along with the bumper.
If your looking for a shop in the USA, check out
http://www.autobodyalliance.com.