Adding on a CPL after the PPL is going to cost you more in the end. you need additional hours on top of the original minimum.
So if you did PPL at minimum hours of 45, the CPL add-on is another 60, that is a minimum of 105 hours. you say 15 hours now, closer to 20 for solo, you will be closer to 60 hours total for your ride minimum, now you are up to 120 hours. the Tax credit is only on the part of the training that goes towards a CPL, so you lose out on anything used towards the PPL.
for a Commercial License from scratch you need 100 hours, all can be credited, and your current training is covered. if they tell you otherwise they are lying. the basic training until past the navigation portion is the same, it is after the Nav things get more convoluted.
The ground school portion is 80 hours in stead of 40 and goes deeper (not close to being deep) into air law, met and nav. the aerodnamics are the same, although as a commercial pilot working you will see a lot more of the situations discussed...
So if the tax credit is important, go straight for CPL from where you are, if savings is important a straight CPL is better than PPL then CPL, but a PPL only is cheaper. you WILL see reduced insurance rates with a CPL, and if you go to a Robinson Safety Course you will get additional benefits.
Harald, Former instructor.