在购物车中添加运费报价的最佳方法是什么？
我的网站的基本流程是，

1.) User selects products  
2.) User is shown cart  
3.) repeat 1 and 2 until User wants to pay  
4.) collect user details ( email, address where items will be shipped to )  
5.) make payment via active merchant  
6.) order is submitted to distributor.

我想插入一个航运信息，使用户可以支付他的购买+航运成本通过积极的商人。

1.) User selects products  
2.) User is shown cart  
3.) repeat 1 and 2 until User wants to pay  
4.) collect user details ( email, address where items will be shipped to )  
5.) calculate the cost of shipment, add cost to cart, show to user;
6.) make payment via active merchant  
7.) order is submitted to distributor.

有没有宝石或插件来帮助我这里，如果没有...我怎么才能做到这一点？