ALLAAH is the LORD of the Heavens & of the Earth
The LORD, most High has commanded us to earn and live with halal income and halal things. He prohibited us from earning haram income ( like working in interest-banks, insurance, other…) and haram things.
As coming to the Question it is permitted to purchase products (which are halal) which have discount upon them, even this day(Valentine’s day). Because we are purchasing which are only halal but one should avoid purchasing products which are related to this day even tough the are halal like buying red roses or hearts or anything which are related to this day.
ALLAAH knows best