Thanks to many frequent flyer miles, I will be taking the kids to visit the grandparents this summer. We will be landing in Frankfurt and it would be best if we had a car (we will be visiting family in the Netherlands and Germany). Our dates are 25 May to 23 June. I have researched and played with a few rental codes and these are the best prices I have found:

$806 economy car with National using code
$806 for compact wagon with AVIS going off airport in Frankfurt

Autoeurope will match National, I have not asked about an AVIS match yet.

Any suggestions? I have tried the majority of codes and very few work in Europe. I know that National will stack codes, but I have only found the one (Emerald Isle code) that will work.

I know these are pretty good prices for a month, but I need to save as much as possible.
Thanks!