I want to setup shipping so that the more a customer spends in the shop the less they have to pay for Shipping. 

To achieve this, you would most likely want to setup shipping costs based on "Percentage of price",  by "Number of Products" or by "Price"

In this example we will setup "Calculate Shipping charges by Price" - initial shipping cost is $10, but if you spend $50, the shipping cost is reduced to half price and if  you spend over $200 then shipping is free. This will apply to local region.

1. Click on Central dropdown button => Shipping Charges

2. Choose "Enter my own shipping Charges" and select the shipping method this pricing will apply to.

For the purpose of this example I have created a Shipping method called Standard.

3. As soon as you enable a shipping method, you will be asked to "Setup the regions you want to ship to", i.e. Local Region and/or Rest of the world (outside merchant country)

- Select the countries you want to ship the products into and click OK

- If you are prompted, "Will this method be available for all products?" - click Yes / No.

4. Click NEXT

5. At the “Apply Rates to:” – select “From - To Ranges”

Now you set up the charges in the window below

Then choose By Price in Calculate charges

Choose From - To Ranges in Apply Rates to

Click on the + button to add each range.

Range 1: From 0 to $50 shipping is $10 - (if you spend less than $50 then shipping cost is $10)

Range 2: From $50 to $250 shipping is 1/2 price - that is $5.00
(so if you spend between $50 and less than $250, you only pay $5.00 for shipping)

Range 3: From $250 and over you get free shipping ($0.00)
I added From $250 to $1000 (you can make it some large amount), shipping is $0.00.
This means that if you spend $250 or more and up to $1000 shipping is zero

6. Click Finish to submit the charges.

7. Save and publish your shop.

Note: You can also set this up to use "buy more products" pay less for shipping.
In this case you would choose the option "by Number of Products"