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BTW/VAT in your case maybe a result of OPC's "single line tax display" but you may consider enabling OPC's dynamic line displays so you can see actual values of the individual calculations (at OPC Taxes and Prices tab), so in case you have 2 vat rules they will show in 2 lines.
there is a reported issue with "billTax" on latest VM 3.4.2 which calculates it a bit differently. When using VAT tax, make sure the type of the calculation is set this way in VM.
other advises for proper VAT config:
- i suggest to create hidden categories for multiple VAT rates
- associate those categories to calculation rules + products so it's clear which product is associated to which tax
- make sure you use "use default rules" on every single product + shipping and payment method otherwise tax restrictions won't be used
we also distribute our "EU VAT and MOSS plugin" together with OPC which tries to calculate VAT for products the best possible way (taking into consideratin VAT number, 3rd countries, etc... )
please let me know at our email if i can help you with this.
best regards, stan, rupostel.com