Shouldn't it be: Usi un piatto, per favore. "Please, use a plate" is directed to you, not to him/her, right?
I looked up the imperative, it seems correct: for -are verbs to make somebody do something you use the lui/lei form instead of the tu form. for -ere and -ire nothing changes. I agree this comes unexpected.
usare has imperative forms usa (informal) and usi (polite). There is no context in the Duo example so no basis for one rather than the other,
The reason I'm confused is I had this question before and it read "per favore usi un piatto", now it says "usa". If "usa" is the imperative, is "usi" still correct then? Are they both correct or does one question need to be reported?
For -are-verbs, like usare, the imperative is a bit weird. "usa" is informal while "usi" is polite (like with "scusa"/"scusi"). Of course, both translate to the same in English, since there is no grammatical polite form, but you have to remember this when speaking Italian.