1. What Do I do If I want to change the order amount before the Order Is Invoiced?

From the Service record, click on the "Related" tab and click on the order: 

Click on View All and then click the button: Edit Products. Then overwrite the unit price (this is the pre-GST amount):-

2. What if my customer has an account with us and they want to pay for the service using a Credit-Card?

You can still create the service from the same regular account. The only difference being that you select the required payment method when you're completing the restocking payment process.  

3. What if this is a one-off customer?

It's standard practice in Salesforce to create an account for customer's who want to transact with us. Reason being, we can carry out marketing for additional products against an account in SF (e.g. for training courses, etc.)

4. What if I want to email the service confirmation to +1 recipient?

You can do this when you click the button "Send Confirmation" on the service record. 

Select Email method and click run: 

You can then add the additional emails in the fields:- Additional To / CC / BCC. 

NOTE: Only 1 email address is permitted per field.