First question I need to ask is how much do you want to spend. The reason being that, sure, you can get a 2X barlow for less than $30, but with the eyepieces you get as standard, you don't really get the image the scope is capable of. If all you can afford is the barlow, go for it. Better that than nothing.
If you can afford a little more cash, not big bucks, you can improve your eyepiece collection, and not be fumbling around in the dark with bits and pieces, only more stuff to drop. I'm an advocate for eyepieces, not barlows, though I do admit they have thier place.
If you would like me to recommend you a few, please PM me. This is not the place for eyepiece discussion.
You have a fine instrument for observing the planets. Marvellous scope, I really like it for observing them. It deserves the best you can afford and you donot need to kill the piggybank for.