Custom Amounts , Merchant Initiated Amounts?

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Do you have any turn key solutions that we can use , I have been trying to work with Shopify on customers custom carts. In essence we want the merchant to intitalte the amount with each customer . I was told this is an easy setting
Here is what we are trying to do 
Have 2 options in the shopping cart
1 - Labor
2 - Parts 
And then we would want to be able to manually enter the amount into the shopping cart?
I.e, for example, Joe Smith comes into the auto repair garage and needs $200 worth of work to the car. 
A would like to 1 - Labor $125 manually enter this amount on the shopping cart and 2 - manually enter part $75 in cart
Deliver would not be needed as it's in-store 
Any Solutions?