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Thank you for coming to Just Answer and allowing us to help you with your question. If the postage stamps are from the United States and were purchased in the last few years, then the stamps printed without a postage amount on them are called "forever" stamps and they were made to be usable forever for a first class letter (a standard letter) regardless of the current cost of postage. The forever stamps were designed so that even if the amount of postage increases, people don't have to mess with buying cheap stamps for the amount of the increase. With the forever stamps, people don't need 5 cent or 3 cent or similar cost stamps to make up the cost difference between the price of postage when the stamp was purchased and the cost of postage if the stamp was used after a postage rate increase.
I hope this answers your question. If you need any followup information, please let me know and I will be happy to explain further.