Is it permissible to buy a pin with the name of a deceased grandmother on it and place it on a bag to remember her and supplicate for her?

There is no harm in reminding oneself to supplicate for the deceased by writing down their name or similar actions. This is indicated by what has been narrated from the Salaf, such as the action of Ziyad, the freed slave of Ibn Ayyash, who used to supplicate for those who helped him gain his freedom until he died. As for the desire to feel that the deceased is with you everywhere, there is no basis for it.