What are the ways to draw in the online store?

Question

Online shopping

I have a slightly unusual question, primarily directed to developers, marketers, and other online store workers.
The task is to draw prizes (fair draw). Those. there is a customer base (who placed an order and signed up to participate in the drawing) and you need to draw a prize among them. By the principle of chance, i.e. if it were, say, an ordinary retail store, they would throw pieces of paper with the names of customers into the drum and pull out the winners. I must say right away that the option of holding a draw in the same way with filming the event on camera and broadcasting on the Internet disappears for some reason, it makes no sense to voice them.
Those. the task remains to somehow randomly select the winner, but in such a way that the participants are confident in the result and there is no suspicion of fraud.
Sometime a couple of years ago we did a similar drawing and used the following scheme: a special formula was announced in advance that calculated the number of the winner. The formula involved such values ​​of variables independent of us as: the dollar exchange rate (on the day of the drawing by the central bank), weather, the number of participants, the sum of the numbers of participants, and other data. All this data was mixed into a specially invented formula, which gave a result that, as it seemed to us, was rather random. The formula was invented for a long time, tested, and everything was quite honest. Although, of course, a good mathematician would probably find vulnerabilities and prove that some numbers had more chances to win, others less. But that's not the point.
Now we have the task of holding a draw again. We are considering the option with the formula, but I would like to come up with something else. So I want to ask if anyone has experience in conducting such draws, according to some algorithm that is transparent to customers and does not allow cheating? Or maybe someone has some ideas?

Answer 1

If $5 is not a big deal then there is www.random.org/draws