解释这段英语的意思.
这段话的意思（不要单纯翻译）.
Please,send your work using only regular mail.According to the Ukrainian law,no package to a private citizen is subject to the postal and delivery charges,as log as its estimated value is under $20.Please,do not send the packages via express mail (DHL,American Express etc),since packages containing documents only (posters are NOT considered documents) are exempted from the otherwise required customs charges,if the package is send using these postal services.Executive committee will not be able to afford those charges,collected upon delivery,if the express mail services are used.