Airasia even charges 250 Baht for payment fee if you pay with their own Air Asia credit card. Additional charges are normal practice here. The first price for the flight is never the final price.
I once had a discussion with a lawyer for counterfeit products. He told me that for low priced items it is not worth for the fake producers to offer those. Original melatonin is not that expensive. Of course there might be exceptions but this makes sense.
I think that it is not longer possible for foreigners. I was refused to purchase it at different stations (I do not have a pink card and was also not asked for it}.
It is a prepaid Visa card. You have to download the MRT app and register the Visa card (a bit complicated). Within the app you can top up the card by QR Code.
What you need is a dermatologist who has knowledge about skin cancer. Not every dermatologist has this knowledge. Especially because skin cancer is a farang problem Thais do normally not get it. Their skin is better for tropical sun exposure even they avoid the sun.