The sum of a percentage of all items should be the same as a percentage of the sum, no?

Suppose you buy two items costing x and y, and there's a constant sales tax of t (say 10%, or 0.1). You'd pay t * x + t * y, or t * (x + y). You can even generalize this to Σ(t * x) = t * Σx (for x ∈ X, where X is the set of prices you're paying).

In other words, yes.

In case you want the math name for this property, it's the distributive property.

~~I think the issue they were bringing up though is that tax is not applied equally to all items, and that tax may be determined by number of items sold. I don't actually know if this is true or not, but if it is, the distributive property doesn't apply anymore.~~ Edit: I re-read the comment, that doesn't look like what they were saying actually. Either way, if tax is weird like this, distributive property may not apply anymore.