Not so. The only stipulation was that you must be able to guess a number in any range (which is not possible in practice). It doesn't matter that the probability of picking any particular number will be 0, as long as the probability of picking any one between the two in the letters isn't 0. Any time your guess hits the correct range, you win. Any other time, it depends on whether you got the big envelope or the small one.
So knowing just one endpoint of a range is, oddly, evidence as to which way the other endpoint lies - as long as it might be either one. You just take a guess that depends on the number you know in such a way that the smaller the number you see, the more likely you are to guess it's the smaller of the two, but are never certain. Presto, guaranteed you'll get more than 50% right, as long as you get fed both the small and the large envelopes in equal proportion.
If you can't be sure the other guy is playing fair and might favor giving you the low envelope, you need to look at something like the Monty Hall problem.
So knowing just one endpoint of a range is, oddly, evidence as to which way the other endpoint lies - as long as it might be either one. You just take a guess that depends on the number you know in such a way that the smaller the number you see, the more likely you are to guess it's the smaller of the two, but are never certain. Presto, guaranteed you'll get more than 50% right, as long as you get fed both the small and the large envelopes in equal proportion.
If you can't be sure the other guy is playing fair and might favor giving you the low envelope, you need to look at something like the Monty Hall problem.