Just a personal preference thing, I think it might be wise to avoid using "free" in a money context here. Although its meaning is not ambiguous here at all, as "free" can mean "libre" of course but also "gratuit", I would go for "optional" just to be absolutely clear.

Examples :

1 - b) Agreement--mandatory and optional payments. Subject to the provisions of paragraph
(d) of this section, any State may enter into an agreement with SSA ...
(www.ssa.gov/OP_Home/cfr20/416/416-2005.htm)

2 - This Plan Feature provides a convenient method of investing and optional cash payments
in additional shares of Sovereign's common stock without payment of ...
(www.sovereignbank.com/investor/stock/drip_high.html)

3 - Can I buy shares directly from the company? May I purchase
additional shares through optional cash payments? ...
(www.gannett.com/street/sharefaq.htm)

4 - each investor may enrol in the DRIP and begin
making investments with reinvested dividends and optional cash payments. ...
(www.moneypages.com/syndicate/stocks/cc-drips.html)

Another possibility came to mind "voluntary payments", but then in my experience that word is more often used when talking about contributions to funds (retirement, medical etc...)