If/then script needed so a text field is automatically filled in if a specific checkbox is checked

I am making a simple fillable acrobat pdf form. I would like a text block to automatically be filled with a number (specifically, "$8.00") if and only if a neighboring check box is checked. I surely would appreciate any help. By the way, the name of the checkbox is MailingCheckbox and the name of the text field is MailingFee. Below is a screenshot of the specific area of the form. Thanks in advance!