Trump's itinerray shows where he was on the dates the checks were signed.

Who was the courier that brought Trump the invoices, voucher and checks to check?

https://en.wikipedia.org/wiki/List_of_presidential_trips_made_by_Donald_Trump_(2017)

Did Trump sign the checks, or did the Trump Org accountants had a "signature stamp"?

Was it customary for Trump to sign checks? Some? All? Or just these?