USPS Priority Mail International:
Priority Mail International (PMI) provides customers with a reliable and
economical means of sending correspondence and merchandise up to 70
pounds to over 190 countries and territories worldwide.
USPS First-Class Mail International :
The most economical way to send letters, large envelopes, small packages,
postal cards, printed matter, and small packets 4 pounds and under, worldwide.
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Both are what we call "Air mail" as what we call "sea mail" is now outdated
and not used unless the item is way to big and needs to be sent with all the
other bigger items in the shipping containers or unless asked for.
Both are approx. 6 to 10 business days, its just one is for small packets and
the other is for parcels.
Hope that helps and 100 cards for US$6.00 is pretty god damn good. Hope
they are packet properly and not just stuck in one of there flat rate letter
envelopes, some people tell you they will even open a sealed box and pack
them in flat rate envelope to save you money.