the only option to give you peace of mind would have been to book (or stil book) a flight out under the amnesty arrangements, no one knows what will happen on it before the 31st if July but so far the info has been that everyone overstaying their entry or extension stamp is on overstay without pay or consequences - which is great - so, again, the government gave or gives everyone time under the amnesty to arrange their departures to their home countries if there are no other options available for those individuals.
You can try Fastship.co out of Thailand globally, used them 3 times for 1x letter and 2x parcels to mainland EU over the last 3 months. Gives you the courier options, timeframe & price in advance. Worked fine, albeit it’s a bit more expensive then regular mail.
I’m also receiving orders from EBay normally from overseas so there are working options.
Thai Post/EMS also accepts mail out now to most countries, best to check at/with the post office physically.
Yea let us know if you had an actual lab test certificate for COVID19 (hard to obtain anywhere in the world due to shortage of testkits) or just a medial cert that you were free of symptoms?