Can anyone please tell me what is the best way to mail our fresh leaves? (mango guava or papaya leaves) Thanks?

I am not sure yo9u can mail agricultural products to the US from overseas.
If you are doing it domestically I’d use a vacuum sealer like Seal-a-meal They are not expensive ~$40–60 and they work!. The bags are a little pricey. Figure $1.00 per bag but this varies quite a bit depending upon which system you get.
After they are sealed I’d put them in a padded envelope or box.

I know you cannot ship to several states ANY agricultural products like Hawaii and California.

As “fresh leaves” – no preserving, pickling, drying, etc., you would be prohibited from sending them to many parts of the world, as noted above. Even vacuum sealing would be disallowed unless your method of sealing would also kill or inhibit the growth of unwanted passengers. (For example, sending goods sealed in a pure nitrogen environment may be permitted – but you’d have to check with the various countries’ import requirements to be sure.)

This applies to foods as well as other “natural” organic products. You can’t even ship goods on “raw wood” pallets to the USA any more. The wood has to have been treated first to destroy the possibility of shipping unwanted insects or other pathogens to the port city.

Obviously, this is not a blanket prohibition, as the USA imports fresh foods from all parts of the world, and without restrictive packaging. Knowledge of the USDA requirements would help in shipping to the USA; I’m sure that other countries have their own restrictions and needs.

In case it’s not clear, let me add my voice. There is a very long list of areas of the US and whole other countries to where it is illegal for you to send any type of plant material through the mail. To be safe, simply do not send any type of plant material through the mail.

It would be illegal to send them to Australia too. Where are you, and where do you want to send the leaves? A bit more info will help us give you more targeted advice.

You’d have to deal through a fresh products and produce distributor. They would know who the leaves would have to be processed for export if it was allowed at all. Look for a distributor. You won’t be allowed to do it yourself.

@cazzie the OP would have to get certification for all his exported product (in his country), the distributor is only a middleman, a warehouse at best.

The distributor knows better what’s involved in exporting than this OP.