Auto Fill in Customer Section

Dear team.
can we have the option of Auto Fill in Customer like Due Date
image

default form is used to apply on all Sale Invoice.
but some customers required special services so we can apply special charges
in customer section.
Cloud Version

I assume you do not really want to specify an absolute/ fixed date as a form default.
I think that auto fill would require Manager to support relative dates which it currently does not offer. Datepicker Defaults - Feature Request - #2 by Patch

its not about the due date, want to fix special service charges to some customers
dont want to miss those charges.

did you try form defaults?

in Sales invoice default form,
how can you fix Special Service Charges to customer XYZ by default?

You cannot. Manager is an accounting program. It does not manage special contractual arrangements with individual customers.