Ok, I don't think the title does this much justice, I struggled to thing of something to describe my problem in one sentence.

Basically the shop that I am setting up with zen cart is that for a mail order photo processing service.

It is currently set up using the bulk discount option so for example if you have 10 negatives to scan to CD they are priced at 30p each, or if you have 100 to scan they are priced at 15p each.

There is an option for having the images scanned as JPEG (default) or TIFF (5p extra per negative).

Nice and simple so far.

What I am having problems with is the option to add multiple copies of the CD's at say £2.50 per extra disc, the additional CD's would be £2.50 extra each if you were sending in 10 negatives or 1,000 negatives, but obviously if you added 1,000 negatives to the order and chose to have an extra CD it would add £2.50 on 1,000 times.

So this is what I am attempting to achieve:

Negatives: Priced by bulk discount.
Upgrade to TIFF: £0.05 per negative
Additional CD: £2.50 extra each (no matter what the quantity of the product you are ordering)

Is this impossible, or have I side stepped something simple?

Regards
Jim