Royal Mail has imposed a cap on the number of stamps every shop can buy to ensure it benefits from price rises later this month.
The price of first-class stamps will increase from 46p to 60p and second class from 36p to 50p from April 30.
The postal service has capped supplies before the price rise to 20% of each retailer's annual allocation in a bid to "protect revenue" and prevent shops stockpiling and cashing in on the higher prices.
Royal Mail spokesman James Eadie said the revenue from the increase was needed to maintain the six-day postal service.
"We do have a sensible allocation in place so that individual retailers can pre-order in advance of the price rise, based on their normal full year expectations of demand," he said. "These allocations are in place for all retailers so we can balance the customer demand with the need to protect Royal Mail's revenues. This is a prudent and appropriate policy. Our priority is to ensure that the proceeds from this much needed price rise go to sustain the six-day-a-week service which has been loss making for some time."
The postal service said it would not comment on reports of individual shops running out of supplies but said: "What we can confirm is that we always seek to strike the right balance between meeting the demands of our customers and protecting Royal Mail's revenues immediately in advance of a price rise.
"Individual retail chains are given a reasonable and proportionate allocation of stamps in advance of any price rise. How they then decide to use that allocation is a matter for them, particularly if they choose to discount those stamps as part of their overall retail proposition. We will continue to discuss stamp supplies with any retail customer which feels it requires more to meet any real customer demand."
Demand for stamps surged when the price increase was announced last month but has since fallen back, Royal Mail said.
Mr Eadie said: "There is a good supply of stamps across the country. We have more than adequate stock in place to meet customer demand. Stamps are available from a very large number of outlets across the UK - approximately 45,000 in total."
Ian Murray, the shadow postal affairs minister, told the Telegraph:."The disproportionate rise in the cost of stamps will have a significant impact on small businesses and those on low incomes. It's understandable that those who will bear the brunt of the increase will want to stock up before the price increase."