可以先看一下Wikipedia中对Maintenance fee (patent)的释义：Maintenance fees or renewal fees are fees paid to maintain a granted patent in force. Some patent laws require the payment of maintenance fees for pending patent applications. Not all patent laws require the payment of maintenance fees and different laws provide different regulations concerning not only the amount payable but also the regularity of the payments. In countries where maintenance fees are to be paid annually, they are sometimes called patent annuities.

参考译文：维持费或续展费是为维持已授予的专利而支付的费用。一些专利法要求为未决的专利申请支付维持费。并非所有专利法都要求支付维持费，不同的法律在应付金额方面和在支付规律性方面都有不同的规定。在每年要支付维持费的国家，有时称为专利年金。

根据wiki的释义来看，题主所说的“维持费”和“延展费”译法都对。