Can I put a UPS on the shelf?

Potentially, but check the particular UPS rather than treating "UPS" as a size of equipment.

It needs to be within the 15kg shelf rating, fit stably on the platform and be allowed by its manufacturer to operate in that orientation.

Many UPS units are dense, heavy things and larger rack models have proper rails or support hardware for good reason.

If a UPS is already flirting with the shelf limit, I would use the support system intended for it rather than constructing a makeshift compromise.