The Royal Mail web site lets you work out how much a letter packet or parcel will cost to post.Just weigh the item on the scales in the kitchen, then log on to the Royal Mail site and type in the weight. You then get the prices of the various options.
Then of course put on stamps, or go to the Post Office and buy them knowing in advance how much to pay. This will save time in the queue at this busy period.
You can of course buy the stamps online to print yourself, but much better to support your local post office!