Wigglyamp, you are obviously an avionics shop (perhaps the one mentioned) and no I don't see a problem with you charging £600 to other avionics shops for the use of the approval you obtained.
Well.... except that the other shop will mark it up to £1000-£2000
I have had quotes for £2000 from shops who would spend a few hours preparing stuff to send off to derassociates.com who would have charged the shop $300.
The problem lies in the wider issues and we will have to disagree there.
The FAA clearly allows FAA installers to use their experience. Of course a GNS530 install in a King Air 350 will be different to one in a PA28
It is implicitly assumed that an installer working on a KA will know how a KA works.
This assumption could of course be wrong. One could have a cowboy working on the KA. But the FAA, who looks after at least 90% of the world's GA, has wisely decided that it cannot regulate-out outright stupidity.
And anyway a "cowboy" will just do the work and not get the approvals, or not make a logbook entry at all. A huge amount of work is already done in Europe without logbook entries of any sort, and always has been.
For some reason EASA thinks it does need to regulate for stupidity/illegality, which is undoubtedly appropriate in some cases... but does EASA know something we don't know, e.g. European avionics shops are mostly incompetent and need to be protected from themselves? I know US avionics expertise is generally much better than here, but the difference can't be that great, surely?