Yes, you can affix a sticker or a label before the goods are put on the shelf in the store or at the checkout. The texts (article 15 of the AGEC law and the decree n° 2021-835) do not bring any precision on this subject, all situations are allowed.
Indeed, the proof of affixing the «info-tri» sorting will only be assessed by the control authorities (DDPP/DGCCRF) in the store. They will not monitor the presence of the «info-tri» sorting later on in the garment circuit.