You might consider using a static VPN tunnel between the main office and the branches. We use a commercial product that supports this between our main and branch (singular) office. This way, it looks like the networks are always connected between each other--no need to initiate a connection. Ours uses IpSec.

It is possible that you might be able to get a firewall capable of this for < $250; thte ones we use do other things (like the mobile VPN as well as the static) and are in the $500-$1000 range.

I didn't look at OpenVPN, I will confess. It's price point is likely better for the non-profit.