> select from orders table where basket size is the biggest. We will then email a nice thank you note to this customer and attach a small coupon/voucher.... ...

> select from orders table where basket size is the biggest. We will then email a nice thank you note to this customer and attach a small coupon/voucher.... ...Guess what? 99% of these people became repeat customers Sounds like you definitely need some ML there, in the form of statistics. Was there a difference in probability of repeat customers between sending and not sending the voucher? Was there a difference between basket sizes and probability of repeat customers? Is there an interaction between the two?