@Ni**
you'll be fine, just mail it out like you planned.
Keep in mind though it's taking up to 6 weeks to get the receipt back because while you mail your 90 day report to Chaengwattana, the 90 day reporting is out at Muang Thong Thani and they have to take them out there, enter them into the system and then mail them back to you.
Don't freak out if you don't get the receipt back in a couple weeks. Keep your EMS tracking receipt to show when they received it (just in case you don't get your slip back)