Rates.

The denominations could be used alone to prepay for the following services:

Denomination Prepaid letter up to 30 leagues Payment on delivery up to 30 leagues Prepaid letter over 30 leagues Payment on delivery over 30 leagues Newspaper
½ real Under ½ ounce       1 to 15 ounces
1 real ½ ounce Under ½ ounce Under ½ ounce    
2 reales 1 ounce ½ ounce ¾ ounce Under ½ ounce 1 to 4 pounds
4 reales 2 ounces 1 ounce   ¾ ounce  
8 reales   2 ounces      

One league = approx. 2.6 miles = approx. 4 kilometers. 30 leagues are therefore about 78 miles or 125 kilometers.

It was soon realized that the low rates that had been set were insufficient to cover the costs of the mails. New rates were introduced on December 20, 1856, after not even 5 months of stamp usage. The rate change caused the ½ real to lose its significance for prepaying letters, and the 1 real was replaced by the 2 reales as the most commonly used stamp.

The new rates caused the stamps to be used alone as follows:

Denomination Up to 16 leagues Over 16 leagues
½ real    
1 real Under ½ ounce  
2 reales ½ ounce Under ½ ounce
4 reales 1 ounce ¾ ounce
8 reales 2 ounces 1 ¾ ounces

The newspaper rate was set at ½ real per pound. Prepayment became mandatory with the new rates.