Review the item:
Watching the Home Shopping Network late one evening, Clarence saw handheld
televisions advertised for only $49.99,
so he bought ten of them, in his lifetime, Clarence was certain that
he would need them all.
The comma between $49.99 and so is correct. Clarence saw handheld televisions advertised for only $49.99 is a main clause. He bought ten of them is a second main clause.
A perfectly legal move in writing is joining two main clauses with a comma and a coordinating conjunction like so.