Magento flat rate shipping with incremental product cost?

Question

I'm trying to find an extension that will let me do the following:

1. Per-category (preferably) or per-product (ok) settings where flat rate shipping applies for the products in that category / product.

2. Enter a base rate for the first purchased item and then a different cost for each incremental item.

I am finding lots of extensions that will only let me set a single flat rate, so when you order 2 or more it just does a straight multiplication, which would be wrong in my case.

Answer 1

In native magento algorithm of calculating flatrate is following:

Mage_Shipping_Model_Carrier_Flatrate

$shippingPrice = ($request->getPackageQty() * $this->getConfigData('price')) 
- ($this->getFreeBoxes() * $this->getConfigData('price'));

In other words it's almost equals qty * price

You can modify existing class or implement your own carier, based on this. Creating new carrier can cause more time, so if you don't need base flatrate - just copy this file to local folder and edit the algorithm

You can put here any condition. Get product's category. Or implement smth like this:

$rates = array(1 => 20, 2 => 35, 3 => 45, /* other rates */)
$shippingPrice = $rates[$request->getPackageQty()];

If you have troubles with algorithm - share more details and we will help.