For every CHF 20, you receive 1 sticker in Migros supermarkets (grocery), at VOI (Migros partners) and through Migros Online. The checkout staff is informed and issues the exact number of stickers based on the actual amount of the purchase. If the amount is then less than CHF 20, no sticker is issued.
It is irrelevant whether it is a difference of cents or CHF.
If you have further questions about the collection promotion, you can use the following contact form: https://help.migros.ch/it/contatto.html
We're sorry we can't give you a more positive answer, and we still wish you lots of fun with the Migros promotion!