"He orders more tomatoes."
Translation:Li mendas pli da tomatoj.
pli is an adverb, not a determiner or counting word -- it can modify a verb, but not directly stand before a noun like that.
It's a bit like "a lot" in English -- you can't say "He orders a lot tomatoes", even though you can say "He sings a lot" (adverb). Before a noun, you need "a lot of tomatoes" with the extra word "of" in there to turn it into a measure "a lot of".
Similarly here, with li mendas pli (he orders more: adverb) but li mendas pli da tomatoj (he orders more tomatoes: measure).
- pli = more (an adverb), e.g. "he drinks more"
- plia = more (an adjective), e.g. "more coffee" in the sense of "additional coffee" (plia kafo)
Just add the adjective ending -a.
Then once you have an adjective, it can take further adjective endings such as plural -j and accusative -n.