My advice would be to see an accountant before you do anything else as you really have no idea how GST works.

If you are a registered business with an ABN you would be filling in a BAS every month/quarter.  That is where you should be declaring and paying your GST, less the amount that you are claiming back.  It is a 2 step process in the BAS but has the effect of you paying just the difference between the 2 amounts.

I would advise not registering for the GST any sooner than you need to as it involves a lot of monthly paperwork that is not necessary unless you have to.  You have 21? days to register after you turn over $75,000.

BTW, the GST included in a sale of $139.95 is $12.72, not $14.00...another reason to see an accountant to make sure you understand your liabilities.