What did the manufacturer mean by asking if an angle should be made for the fingers?

When they say 30° degree, I think they mean to chamfer the end of the edge connector for easy insertion into the C64 cartridge slot.

When they do that, you will probably lose the last mm of copper on the end of the fingers.

If they do this free of charge, I see no reason not to say yes please.

Image from http://www.industrial-electronics.com/et-4e_14.html

It is my understanding that they are asking if you want them to bevel the PCB where the connector is located.

Here is an illustration from Eurocircuits demonstrating the end result:

If you look at other card edges you will see that they are almost always beveled.