When sending packages via USPS, you must pay the shipment costs in advance. Stamps may be used in every circumstance, but for larger shipping volumes, they may not the best option because of the labor to affix them. There are, however, other options. 

Postage Meter To use a meter, you prepay a balance (by phone or online), which is reduced each time you use the meter.
Click and Ship Print online labels on the Internet USPS site and pay via credit card.

USPS Shipping Assistant Software — Global Shipping This software is free but is not as fast or flexible as some offerings. For lower volume it works and can work with an IBI (Information Based Indicia) postage meter. You can visit
www.usps.com/shippingassistant/ibimeter.htm for more info.

Mailing Systems These systems, such as Pitney Bowes, offer reduced mailing rates. 

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