The PayPal Shopping Cart feature can calculate postage based on the total price only.

ie. Price Range = £0.01 - £10.00. Postage Cost = £1.00
Price Range = £10.01 - £20.00. Postage Cost = £1.80
etc.

Seems like it may be suitable for your needs....

However, I used to use PayPal Shopping Cart and it works fine but looks extremeley amatuer, like you've knocked something up in about 5 minutes, which will reduce sales dramatically.

You would need to hard code every page of your site and every "Add To Cart" button. You would have no attribute options unless you used more than 1 Buy Now button. For an item with 10 colour options the page could soon become over-run with buy now buttons and swatch/product images. There are no features to email customers, you would need a seperate mailing list which, for professional looking emails, is likely to cost you money.

For the more professional look you would need a more professional solution, such as Zen Cart. Attribute images are available so when someone selects a different colour, a different image is displayed and always only 1 buy now/add to cart button.
Spend an hour learning the attribute features in Zen Cart, there are tutorials readily available and you could pm me or someone else if you need help.