How to enable cod for selected countries?

This a flat rate plugin, not cod

how to rate a multiple quantity charge.

eg: for 10 quantity, for the 4 quantity charge is 10.

then for the 10 quantity = 10/4 =2.5 => charge = 3*10 = 30

The plugin does not support what you’re trying to do.

Is there any solution for that?

Not with this plugin. Thanks.