Can I post a purchase invoice direct to a sales code in Quickbooks and how would it show up. Would it reduce the sales total?

Could you clarify what you mean by "sales code"? If you are trying to adjust the sales tax on an invoice, you can edit it directly on the transaction screen in the Includes VAT fields.

However, please note that adjusting the sales tax amount will not reduce the total amount on the sales invoice. The subtotal for your goods or services remains unchanged.

QuickBooks calculates VAT automatically based on the specific percentage rate assigned to that tax code. Manually changing the tax amount overrides this calculation for that transaction, but it will not lower your net sales.

Please let me know if you have any further questions about VAT in QuickBooks Online. I am always here to help!