Our system is built to assume a constant VAT rate per product so it's possible we would have to do a huge overhaul of the system for it the user to be able to create some "VAT period"-like behaviour. We're evaluating our options and working on this right now.
In the meantime, we encourage you to use the Amazon VCS service as a workaround for a short term. Amazon will calculate the VAT rates for them, and then send us the information.
Please note that as soon as Amazon gives us the tax information, VAT data that you entered in the Cost Manager will be ignored :)
More information on how Sellics calculates VAT can be found here.