First machan, use a safe web browser.... Either the latest version of Firefox, Opera, Chrome or Safari. Internet Explorer will do too, but i prefer Firefox. Well there is not SAFE way of entering your credit card information in to the website just by using your keyboard.
Well make sure you don't have anyone behind your back trying to steal your information.
The worst case scenario is your computer having a keylogger

This is really bad! Because, if there was any Keylogger, without a good Antivirus/Internet Security suite, you wouldn't know it exists but it can steal ANYTHING you type on your keyboard. So make sure you are using a GOOD Internet Security suite + good antispyware suite. (Most Internet Security suites usually scans for spyware too)
Do a fullscan of your system to make sure that there are no such Key Loggers. If you by any chance feel unsecure, you may use a Virtual Keyboard.
OK now to answer your questions?
1. How Safe?
Very SAFE! Unless you are entering the information to a different WEBSITE! Where you think it is Paypal or eBay but it is actually some other fake phishing website. So beware! Make sure the website is legitimate! Check for authenticity!
So don't worry about eBay or Paypal!
BUT there is an exception! Just because you trust eBay
DOES NOT MEAN you should try the Sellers who sell stuff! Carefully pick your sellers. Check their rankings and CHECK other User comments!
2nd Question
If you are receiving the package by Air Mail; No you usually do not have to pay!
I have ordered laptop cleaning cloths, Bluetooth Dongles and a Phone Screen Guard and it all came in the normal post and i didn't have to pay a cent!