The referral-only approach
With a referral, the client chooses and pays for the software directly, and your firm is invited in as the accountant. It keeps your overhead low and suits firms that do occasional compliance work rather than monthly bookkeeping. Nothing needs to be set up on your side.
The firm has limited influence, though. Clients may pick different products, leave without telling you, or turn to the vendor for help first. Your brand appears only in your own emails and reports. Over time the software relationship belongs to the vendor, not to you.