Looking for a Shipping price change mod

Hey Everyone,

Here's my issue, hopefully I can explain it properly so someone may be able to recommend or point me to a good mod for my needs.

I have some products on my store and well I want to set specific prices for them on the shipping depending on how many items are being purchased at once, heres an example:

1 Mat - £5.00 (shipping)
2 Mats - £5.00 (shipping)
3 Mats - £7.50 (shipping)
......
10 Mats - £14.75 (shipping)

Etc... I want to mainly be able to manually set the price for shipping to update the price when 1 mat is bought, 3 mats are bought etc.. I need to mostly be able to MANUALLY change it rather than just set a % of how much to get a discount off, because the shipping price is completely different depending on the product.

Any ideas what mods to use? Any links to them would be really appreciated, thanks. Or better yet anyone that could paste in the code i need to edit to enable this type of mod would be better. I don't have too much money to be spending on mods/plugins but I desperately need this. Right now my shop just doubles the shipping everytime a matt is added (4 mats + £20.00 shipping (this using the example that i have set £5.00 for shipping each). i dont need to charge £20.00 for shipping, but dont know how to change it.

Thanks