Gardener only mentions net prices

I have checked again. Did not find the original order amount.

That has no relevance at all... no more than the question of whether it is charged as a lump sum or per stone. A contractor must provide gross prices to consumers. If he does not do so, it is simply wrong. Period. If prices are mentioned in the WhatsApp chats, the consumer may and MUST assume gross prices. He does not have to ask and he does not have to wonder about "round sums." Everything else is irrelevant – including whether there was a first order, or how large it was. And concerning the VAT rate: the delivery date applies here! Not the invoice date.