How do I allow the user to select only one of two options

Using JotForm to create a registration form.  If the visitor registers before a specific date there is a fee of $400 if after there is a fee of $500.  I am using the PayPal option.  How can I limit the user to only one selection?  Trying to accomplish what is done here https://joryserota.com/nelson-pilates-registration/ where I used 2 buttons.  Have attached screenshot of what I have so far.  The first 2 lines are as 2 different products but the user can select both - not correct.  The 2nd I created options for the product but I don't know how to enter the separate price for the option so it displays in the total box

Sue

To achieve that, you will have to set Users Can Select Multiple Products option inside of your payment field settings to NO. 

After you set Users Can Select Multiple Products option to NO, your form will look like this.

Hope this will help. Let us know if you need further assistance.