Take a look at the sentence again:
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.
Go back to the sentence to try again.