Good points raised here.
If you are sending final artwork that does not need to be edited any more a flattened bitmap of the layout would be the most bullet-proof way to do this.
There is nothing wrong with jpgs saved at 100% if they are just going to be printed.
Even if you send it to a shop that does use CS, good pre-flight practice would be to simplify the file as much as possible to avoid issues that will crop up.
The printer's CS installation may not have the same assets the designer's copy has, i.e.. fonts, plug-ins, 3rd party fills, custom presets, newer or older version, etc....
All those things need to be expanded or flattened prior to sending it out.
As for the
sign shop using only CorelDraw this sounds wrong.
Corel does not offer a rip program that will drive a wide format printer as far as I know.
They may be opening your ai files to check them but I don't think they are printing from Draw. See if you can find out how they do their work and it will make it easier for you to get what you expect from them.
If they are printing directly from Draw then the advice of finding another printer sounds like a good idea.
good luck
wayne k
guam usa