1a) You should receive an email about the payment details from Paypal.
1b) You should receive an email from Paypal about the payment details.
2) Once Paypal has received the payment, I will send the items to you.
Is 1a) and 1b) both correct?
Is 2) correct?
Is 1a) or 1b) equally correct? I mean is any one of them better English or more commonly used?
2 is fine.
Note that PayPal contains two capital letters. It's a brand name so you should stick to the way they write it.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.