Add Fee if within date range

Sorry, if this is a repeat question, but I couldn't find on the forum. I've used the Date Picker widget to not allow sign-ups within X number of days.  IS there a way to essentially do the same, however, if within X number of days, the form will add a $25 admin fee for short notice?

You could achieve it by adding a second date picker field to your form, setting up the current date will help you to know the number of days. 

Setting up current date as default. 

Once the field has been added, you will need to add a Form Calculation widget to your form, this will help you to calculate the number of days between the current date and the selected date on the "Date You Would Like to Have the Party" field: 

You will be able to add the fee based on the number of days, you will need to use conditions to achieve it and here is a guide that will help you doing it: https://www.jotform.com/help/268-Conditional-Logic-Inserts-Text-Calculation-Into-A-Field 

Once you have calculated the fee you could simply include it in the current calculation you have in your form. 

Let us know if you have any question.