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

i making simple fillable acrobat pdf form. text block automatically filled number (specifically, "$8.00") if , if neighboring check box checked. surely appreciate help. way, name of checkbox mailingcheckbox , name of text field mailingfee. below screenshot of specific area of form. in advance!

 

if box not ticked, should text box enabled user input, or should set 0 (or blank) automatically?

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