Creditor must file application for postpetition fees and expenses under § 506(b) and Bankruptcy Rule 2016 to give debtor and trustee opportunity to oppose. Section 1322(e) requires determination of prepetition fees and expenses in accordance with underlying contract and applicable state law, but postpetition fees and expenses incurred by mortgage lender are governed by general reasonableness standard of § 506(b) and Bankruptcy Rule 2016.