I should put it in this way, that Google didn't take the money yet, just submit an authorization (hold) for the charge. This is to ensure that you are able to pay 100% for the replacement in the event that you didn't return the original.
Once they have received the original unit from you within the allocated time, your bank will release the amount back to you.
It will depend on the shipping service or courier you are using. To ship back to US and back to SG, you will require a temporary re-import permit. Courier service handling international delivery should know the process and able to assist you on this.
You will have to provide relevant documents such as the actual invoice as well as the RMA form, so that the company handling this for you, could produce these documents to the custom. The documents are actually to proof that this is the actually the same unit that has been sent for repair/replacement, and not liable for GST.