If i buy a refurbished macbook with an apple card, is the entire purchase price due at the end of the billing cycle?

Yes. Monthly Installments are not applicable to Refurbished products.

See the first footnote here: link-> Apple Card - Monthly Installments - Apple

 ACMI is not available for purchases made online at the following special stores: Apple Employee Purchase Plan; participating corporate Employee Purchase Programs; Apple at Work for small businesses; Government, and Veterans and Military Purchase Programs, or on refurbished devices.