Help with a code in userform

Variables:
txtHC = textbox Hours of Access (Input)
radA = Package A
radB = Package B
radC = Package C
MC = Mounthly Charge (Output)

• On Package A the charge is $9.95 for 10 hours of access plus $2 for each addicional hour.
• On Package B the charge is $14.95 for 20 hours of access plus $1 for each addicional hour.
• On Package A the charge is $19.95 for unlimited hours of access.

There's also a checkbox to check if the client is from a NonProfit Organization that gives the client 20% of discount on all the packages. The user must enter the hours of access, select a package and the check the checkbox if the client is from a NonProfit Organization or leave it blank if is not. The Output must be the monthly charge.

I don't know how to make possible to add $2 or $1 depending on the package and the hours of access. For example:

Client 1: Package A, 5 hours, nonprofit $7.96
Client 2: Package B, 25 hours $19.95

On the client 2 it has 25 hours of access and it was selected package B meaning that there are 5 hours of access addionals that must be charge $1 each but I don't know how to do that or the 20% of discount.