Paying cash in e-commerce is quite unusual and not very practical. Concerning the sense and the global possibilities of E-Commerce it does`t make sense and it would be unsafe to put cash in an envelope and send the money via postal services.
A better and safe way to pay your e-commerce bills could be the wire transfer oder credit transfer.
Note: Banks get paid for their services, from the sender and the receiver. You have to give your bank an order to transfer money. Furthermore your bank needs IBAN and BIC codes to address the receiver. Your bank sends a secure message to the receiving bank, that there is money going to be send on the receivers account. Sending money needs hours to days.
With a credit card you may purchase goods and services and pay for them without cash. The payment works by direct debiting between the cardholders account at his bank and the account of the shop or the supplier. Usually you have to pay an annual fee for using a credit card, but some banks are offering them for free. Note: if you are in foreign countries and use your credit card you have to pay a commission for using it. When purchasing goods in the internet or via mobile devices you will be asked to tell the seller your credit card number and a verification number located on the back site of your card. 1) See also Micropayment and McDonald`s news - pay via touchscreen in Electronic payment options
A prepaid credit card usually works like a real credit card. There is one difference. Your prepaid account needs to be secured. I.e., there must be an amount of cash on your account. Security advantage: if the amount is gone nobody can pay with this card. http://www.kreditkarten-1a.de/Prepaid-Kreditkarte/Prepaid_Kreditkarte_Zahlung/prepaid_kreditkarte_zahlung.html
This type of paying does`t work in E-Commerce. To use a travellers cheques you have to sign your check in front of the cashier, after you signed it in advance in front of your bank employee. This is a security measure to make clear, that you are the owner of the cheque. When you buy travellers cheques you get a receipt including all series numbers of the cheques. For security reasons you have to store them in separate places. Otherwise someone could take advantage of stolen cheques. http://www.zahlungsverkehrsfragen.de/azvframe.html
Costs: As a commission and for insurance reasons your bank will charge you one percent of the purchased currency-amount.