Dropping off package at USPS collection box count as shipping?

I am new to Mercari and I made my first sale. I’ve already boxed the product and attached the shipping label and planning to drop the package at the USPS collection box tomorrow (Sunday). Does that count as being shipped (so I can mark shipped to the buyer) or should I only mark it as shipped once USPS comes to collect the package on Monday (as they’re closed on Sunday)?