Sending her just a couple of dollars can bring you much more mail and she will be grateful for your help. Better yet include 1 or 2 foreign postage stamps from her home country. They're much easier to conceal in your letters. Russian ladies may not need your assistance as much as the asian ladies. Japanese ladies and ladies from Western Europe do not require help and may very well be in better financial condition than you.
All countries are different as is every girl's financial situation.
Do save all letters and envelopes you receive from the ladies. You never know which one you might marry. Cancellation dates on envelopes and past correspondence may be helpful with immigration. Do save copies of your letters or at least make detailed notes of what you wrote. Writing to many ladies can become confusing.