I assume you're setting your products up as tax inclusive.
Why are you not letting Shopify calculate the tax for you at checkout?
Malaysia has reduced GST from 6% to 0% effective 1st June 2018. And the GST will only be abolished in 1st Sept 2018 and replaced with SST. Thus all products are not entitled for taxation for checkout during this period of time. Customer should not be charged of any tax upon check out.
Currently all Products have a tick to Charge taxes on this product at Product level. Instead of changing all Products at Product level by untick Charge taxes on this product, can we just do the following:
1) At the Setting/Tax settings: Untick the box for: All taxes are included in my prices
If selected, all taxes will be calculated using this formula:
tax = (tax rate * price) / (1 + tax rate). For example: £1.00 at 20% VAT will be £0.17 (rounded).
2) At Setting/ Tax Rate: Change the Country Tax Rate from 6% to 0%.