How good idea do you think is using other method for billing that is not whmcs ,lets say any cart be free or not license but......................
administering all client hosting accounts just with a booklet and a pen?
Depends on how many customers you have. When we were doing design projects for customers we often used an Excel sheet to track how many hours of work we had done etc, but we're talking less than a dozen projects per month.
If you're dealing with multiple clients and those clients are paying monthly or quarterly, do yourself a favor, sped the $15/month for the WHMCS Lease and get it done right. Your clients will appreciate a professional experience also. It's not always about YOU and YOUR busienss - often times things are implemented because it's what your client wants/needs/likes.