Simony, by definition is to sell roles, offices, or titles and ecclesial preferment within the Church.
Stipends, by definition is the defraying of expenses.
The Masses would occur whether or not anyone ever gave a stipend. you can’t “buy” an extra one. The Masses you in which you can have prayers offered specifically for a loved on would happen anyway. You would be praying alone in your intention, in a way, even though we do all pray together for each other and each other’s intentions. When you offer a stipend, all know in advance and during the Mass that there is a special person that needs our prayers.
It’s not the same thing at all. Hope this helps.
Be secure in the knowledge that if it was forbidden in the Bible, there’s no way would the Catholic church embrace it. It is possible that people misunderstand the terms, however.