I do not understand "a zoomed-in selection". But let's try to give you an option.
Prepare an A2 document at the real size and the resolution of your images, so they are not resampled when using this new file.
Make a "collage". For example: As the file is A2, you could make a collage of 16 A6 postcards or 32 A7 size, and send one of these samples.
Use the exact same resolution as the final print.
This way you will have a sample of the exact same paper as the one that will be delivered, the exact same ink and colors, and the resolution.
I am not mentioning details on how to prepare your file, because I do not know how the printing company need to work with the file.
See if you need some margins if you need to crop your file to the exact same dimensions, or even if you need to send a PDF file instead of a JPG or TIFF file.