[welly]

I'm not sure I can help with this Julie, but firstly, let me make sure that I understand the question.

Are you saying that you only want your customer to be able to order certain quantities? Ie, in your example, your customer can only order 15, 20, 25, etc, and not be able to order 17, or 21, and so on?

Also, are the prices on a 'per unit' basis, so for a quantity of 15, the price is $110 per unit ($1650 total)?

Depending on the answer(s), you may be able to use product variants to do what you want. Your variants would be '15 widgets' (priced $110), '20 widgets' (priced $130), etc. The downside is that your quantity selector would still exist, and overall it won't do your stock control any good.

Welly