Tax rate on delivery cost

How does one specify what the tax rate should be on the delivery cost element of an order? We can specify what the tax rate for an order item should be but there doesn’t appear to be any provision for the tax on a delivery charge.

If we provide delivery_cost including VAT then the order total that is calculated in Veeqo is correct but the Tax is incorrect.

If we provide delivery_cost excluding VAT then the Tax is correct, but the order total is incorrect.

E.g.
Order items total (ex vat): £90.
Delivery cost (ex vat): £10.
VAT: £20.
Total (inc vat): £120.

Results in the following if we provide the ex vat delivery cost:
Subtotal: £90
Shipping: £10
Tax: £18 (should be £20)
Total: £118

And the following if we provide the inc vat delivery cost:
Subtotal: £90
Shipping: £12
Tax: £18
Total: £120

In the first case the total is incorrect, and in both cases the tax is incorrect.

We’ve tried providing the tax_total property when creating an order to see if we can manually override the tax figure for an order but this results in a 500 response with no further information.
{"status":"500","error":"Internal Server Error"}

How do we set the tax rate for shipping fees?