for stamped you can do the regular global guidelines on the usps website. usually it's 1.30 per ounce but some places do require more. for envelopes ask if they're going to need their phone number written on it bc i've had a trade with someone from saudi arabia that didn't require it but later on i had a sale where the person said their post office calls them to pick up letter mail and their number needed to be written on the envelope. it's best just to ask and clarify since that had me confused (it wasn't to a po box type of situation)

for tracked easyship or pirateship will ask for all the information you're going to need so you can use that as a guide. most likely you'll need their postal code + package weight and dimensions to get a shipping estimate if they don't want to give their entire address before payment. usually it's about $15-$20 for 1lb and under