Ryanair refuses to issue invoices for flight tickets?

[Bushpilote]

Hi All, is there any reason why Ryanair refuses to issue invoices for flight tickets. Each time I fly I try to obtain an invoice or at least a receipt / proof of payment. In vain. There is always some weird reason why they refuse my request. I suspect they must be running "parallel bookkeeping" and so if (and only if) they issue an invoice they must pay the VAT over the invoiced amount. Whereas without an invoice they can pocket the full amount.
Cheers,
Bushpilote