Quick Opencart Royal Mail Shipping Module Code

I have the following problem with my opencart ecommerce store:

I have royal the mail module active with 3 options for US shipping:

Standard first class,
Recorded First Class
Special Delivery

At the moment, the user can choose between the three, but what I want is to apply the following rules:

Standard first class not available when cart value >=$50
Recorded First Class first class not available when cart value >=$300

Ugrent!
Thanks!