This is the Frobenius problem for two relatively 
prime positive integers.  In this case 5 and 11.

Frobenius theorem for 2 relatively prime positive integers

If p and q are two relatively prime positive integers greater
than 1, then the largest integer which cannot be expressed 
as px+qy, where x and y are non-negative integers is given by
the formula pq-(p+q). That is, the difference between their 
product and their sum.

So the product minus the sum of 5 and 11 is 

5*11 - (5+11) = 55-16 = 39

So you can't get 39 cents worth of postage using only
5 and 11 cent stamps.  However you can get any greater 
amount of postage using only 5 and 11 cent stamps. 

Edwin