Should I write email to POI after admission offer

[slaugh]

I have received an admission offer together with a research assistantship offer in my POI's lab. The email came from graduate affairs office a few days ago. 

Prior to this email I have contacted my POI for some questions and etc. He didn't write to me after the offer (the offer letter came from graduate affairs office with the POI in CC). Do you think I should still write a thank you email to Poi for the financial aid offer? I don't want to sound like I am accepting the offer. However, I also don't want to give the impression that I don't care about the offer, or that the school is not my top choice.

Deadline for responding to the offer is explicitly stated in the email as April 15. But I still fear that my POI will think bad of me if I wait for a month to accept the offer without writing anything to him in the meantime. I still didn't hear back from any other school. 

Edited January 29, 2018 by slaugh

[Elephas]

I think that they will understand if you do not accept for another month and that you are still weighing options. However you could write them saying thank you and if you have any follow up questions now would probably be a great time to ask them so you can make a list of + and - about the current school now and when it is time to see the other response (assuming it is a yes) you'll only have to ask one school those same questions instead of multitasking both at once.