The cost of UK postage stamps is going up AGAIN at the end of the month. If you decide to stock up, make sure you get the ones which say '1st' or '2nd' rather than give the value. That way you're less likely to accidentally underpay next time the price goes up.
Whilst you're in the post office, get them to weigh a submission 'package' with a few more sheets of paper than you usually send, so you're aware of how long a sub you can make without having to pay extra. If you don't pay enough postage editors will probably never see your story and you'll have wasted more than the cost of another stamp.