Pay pal says i can send it through ur email or phone number... if i am reading it correctly.... or it says get the seller to send an invoice like you said..
You go to paypal and choose the "Send payment" option. Put the sellers paypal user email in the "To" box. You can either choose to send the money as a gift, or for payment for services. Paypal charges a fee if it's sent as payment, So the fee will be taken out of what the seller receives. I think it's like 30 cents