If she intended to commit suicide (which she actually did), why buy two boxes of ammunition.
I have a new, recent theory about this based on a comedy sketch by Dave Chappelle that aired on Netflix. In it he describes going to a gunstore to buy a shotgun for self protection and asking the gun shop owner for advice. The gunshop owner grabs two boxes of ammunition from the case because that's what is usually bought.
My theory is that Sol Pais was confronted similarly and not wanting to look naive, bought one box of each.
Here's Chapelles skit - IMO, its compelling.