Adriana is correct--the calculater tend to be inaccurate and I don't use it either. But I mostly sell clothing, which is very easy because it doesn't weigh that much...Weigh it, print Paypal label with Delivery Confirmation (VERY important) and drop it in the mailbox.
You'll NEED a scale, if you have a food scale at home you can use that for for lightweight items, or you can find them on Ebay for under $20. Just remember to ALWAYS get delivery confirmation, it's well worth the extra few cents. If you're shipping Priority go to USPS.com and use the Postal Store to order free Priority mailers and boxes.
As far as recieving payment, just make sure you're email address is correct everywhere and you should be good to go.
I remember the rush of watching my bids go up higher & higher on my first sales...lotsa fun
You can also find a wealth of information (and interesting predicaments) by reading here:
http://pages.ebay.com/community/answercenter/
Good luck!