The €150 threshold is for the price of goods only. However, for cart totals below €150, VAT is charged on goods, duty, shipping, and insurance.

Yes. However, this is not recommended because there are expected to be customs delays for any packages that need the carrier to collect VAT. Also, your customers may not enjoy being surprised with extra costs (duties, taxes, and fees) once their package arrives, which could result in abandoned packages.

Yes, any shipment where the total of goods is more than €150 can have duties and taxes collected from your customer at checkout. You will then ship the package with duties and taxes prepaid and select the option to bill that back to yourself so your customer doesn't get charged again.

Yes, unless they provide you with their VAT number, which will shift the responsibility to them. You will need to include their VAT number on your sales invoice.

Any sales made through a marketplace does not need to have VAT collected by you. The marketplace is responsible for VAT collection and remittance.

No, the carrier cannot act as a tax intermediary.

No, but it would be confusing to your consumer if you don't. If the company is IOSS registered then the VAT must be collected in the checkout and itemized on the invoice. If the online retailer is not registered for IOSS, then the carrier will collect based upon the selected carrier Duty & Tax billing options.