I’m assuming that the invoices are from different suppliers in Austria (as it would seem odd that one supplier would use two different methods), and your customer is located in GB rather than NI. Rules for goods are different in NI.
In preparation for Brexit, many EU suppliers registered for VAT in the UK. Effectively their UK VAT registration imports the goods, so although the company’s head office may be in the EU, the goods are actually being sold to your client from the UK. So the first invoice should be treated in the same way as an invoice from a UK supplier.
On the second invoice, do you really mean self-billing? Self-billing is where there is an agreement in place for a customer to raise VAT invoices on the supplier’s behalf. If this is the case, it would be useful to have a bit more detail about what the arrangements are.
If it’s not self-billing, import VAT will need to be paid at some point. By default this is done before the goods are released from customs, although the courier may pay on the customer’s behalf and charge the customer. Alternatively it is possible for customers to register for PIVA (postponed import VAT accounting). Goods are then cleared through customs without import VAT being paid. The transactions are added to a monthly statement downloadable via the Government Gateway, and the transactions on the statement should be accounted for on the VAT return.