You have the option to enable automatic tax calculation for shipping methods under Settings → Shop → Shipping → [Shipping method] in the Tax calculation section. Automatic tax calculation also applies by default when redeeming discounts and promotions.
This means that the line item for the voucher or the shipping costs can be taxed with multiple tax rates. On the invoice, the line item for the shipping costs is then displayed as follows, whereby it is not apparent which portion is attributable to which tax rate:
The DATEV export from Pickware works with the exactly calculated amount of the line item and is therefore correct. However, this can result in the amount per tax rate excluding the portions of the shipping costs or the voucher on the invoice differing slightly from the amount exported in the DATEV export.