quantity discount for checked attributes NOT product quantity

Hello, I'm using Zencart 1.3.8 and want to price a product in the following way:

Product: Classes, price $100/week (each additional week $80)
Attributes (as checkboxes):
week 1
week 2
week3
week 4

Here's what I want - if you just sign up for one week it costs $100. If you sign up for additional weeks, each additional week is only $80. I don't know which week people will start with, so I was trying to make something that says - if they choose 1 attribute the total cost is 100 but if they choose more than one attribute ad $80 to the cost for each extra attribute.

Is there a way to do this, or a different angle to make it work?

Thank you so much.

You can do this with checkboxes ...

I am guessing you are trying to say: Here is the Class for $100 and each added week is $80 but that I could take week 1, 3 or week 1 and 4 or week 1, 2, 3, 4 ...

Set the Product Price to 100.00 ...

NOTE: this is Priced by Attribute needs to be set to NO

On the Checkboxes you make each of the Option Values
Week 1
Prefix: (nothing here)
Price: (nothing here)
Default Attribute to be Marked Selected: Yes
Sort Order 10

Week 2
Prefix: +
Price: 80.00
Default Attribute to be Marked Selected: NO
Sort Order 20

Week 30
Prefix: +
Price: 80.00
Default Attribute to be Marked Selected: NO
Sort Order 30

Week 4
Prefix: +
Price: 80.00
Default Attribute to be Marked Selected: NO
Sort Order 40

NOTE: I am assuming you have to take week 1 ... I am also assuming the customer is not going to uncheck week 1 as it shows no price ...