How to set conditions when using form or activex controls

I need to figure out how to make my sheet work based on conditions. In the example I've attached I need help with the following:
1. Add a button to be clicked that if $G$2=true, then $C$5=$C$4*2
2. Add a button to $F$5 and another to $F$6. Then if $F$5=true, both of my formulas calculate as they're written already.

Then I need the condition... If the button in $B$5= TRUE, then $F$5 MUST be False.

In simple terms, if we have a customer who chooses a 2 year contract they are unable to use monthly payments and hence that set of boxes must remain "N/A".

Is it possible to create a pop-up to state this that if the button in $B$5 is clicked and someone attempts to click on yes for monthly payments to state this can't be done?

Last question- should I use form or activex controls?

I also want to mention that I'm very much a novice and have never dabbled in VBA code- so please dummy it down to help ensure I "get it". I'm using excel 2013.

Example.xlsx

I'll use whatever is easiest... for me and my limited knowledge base it's easier to keep the worksheet and formulas, but if there's a better way then I'm all ears.

My formulas take care of the calculations, but I'd like to have the user click on the button to trigger the formulas to calculate.

I use if statements though and it relies on the "true" statement

That part I get... However this isn't solving my situation. Maybe I wasn't as clear as I needed to be.

I need the option for monthly payments to not be allowed if someone chooses a 2 year contract. As it's set up now both options can be chosen which is what I need to avoid from happening.

I also need advice on using form controls or activex controls to make this happen. I was using a form control, but wasn't/not sure if that's the best option.