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Hello @sloganwp
Thank you very much for using our plugin. The value of the unticked checkbox (or switch off) is zero. So, you can insert a checkbox field with only one choice and enter the number 1 as its value. In the equation implementation, you can consider the field value 1 as the annual fee, and the monthly fee otherwise.
For example, assuming you have the equation fieldname1+fieldname2, and the fieldname3 is the checkbox field. If the user ticks it (its value is one, annual fee) you want to increase the sum twelve times. The hypothetical equation could be implemented as follows:
SUM(fieldname1, fieldname2)*IF(fieldname3, 12, 1)Best regards.