In order to change your line items so that when they appear in the invoice, their price includes GST rather than excluding and listing it separately, in the Web Portal go to the Settings cog > Accounts > Items.
From there, you can select Edit for the line item(s) you would like to change the GST setting for, and in the Tax Rate dropdown, select the Output GST option.
Please note: If you do not have a GST Output option, or if the tax rate is incorrect, you can add or edit extra Tax Rate options.