There are multiple points where you get more for the same price.

The prices lie between 99*€8.76=€867.24 and 100*€6.57 = €657

So, you can get either 99 or x*6.57=867.24 => x = 132 products for €867.24

but you can also get 100 or x*8.76=657 => x = 75 products for €657

So, from 77 products and onwards, you should be buying more, as to end up in the higher bracket.

I used € because can't find how to get pounds on here.

Also, I realized I did some redundant math, but I decided to leave it there if you might need it/are interested in it.