A lot of competitions can be entered either by email or by post. (E.g. a prize crossword in a magazine that I sometimes enter  you can either cut out the grid and send it in, or type out all your answers on an email).
Assuming one winner has to be picked “at random”, how do they do this? I can think of a few ways:

Toss a coin to decide whether the winner will be a postal or email entry, then choose at random from the relevanty entries. Drawback: the numbers of entries will not be the same for both methods, so (assuming that in the long term, equal numbers of email and postal entries will be chosen) the odds will favour entrants using whichever method gets fewer entries.

Print out all the emails and dump them in the sack with the postal entries, and mix them around. Drawback: timeconsuming and wasteful.

Ignore all the postal entries, or the email entries, and just pick one from the other method: Drawback: presumably contravenes contest regulations.

The only fair way I can think of: count up all the postal entries, and all the email entries. Let’s say there are 1,250 postal entries and 2,100 email entries, for a total of 3,350. Use a random number generator to pick a number from 1 to 3,350. If it’s 1,250 or below, choose a postal entry at random, if it’s higher, choose an email at random. Drawback: complex, and you have to count hundreds of envelopes.
So how do they do it in practice?
And, related to this, when there are promotions like “10 prizes to be won” from random winning packets of crisps, there’s always a “no purchase necessary” chance to win. Now, I’m sure they don’t randomly open a packet of crisps every time someone enters through that route, but if all 10 prizes are inside packs of crisps, where do the no purchase necessary prizes come from? Do the manufacturers have to set aside a certain number of extra prizes? (Edit: I actually asked this second question ages ago, but didn’t really get a factual answer.)