I often book different classes in outbound and return.
last year I flew LHR-SFO in economy and back in CW with Avis car hire added.
I search flight only, choose flights and class then add car at checkout.
I've always been able to pay just a deposit initially.
in fact I’m off to Denver this summer on the same premise although after initially booking economy out, I called up and managed to book WT+ for less than the price I initially proceeded on for WT!!