The mastercard verification software requires a seven digit postal code and will not proceed without it. Canadian postal codes have only six digits. This requires you to book by telephone for an added charge of $25. on Delta. I had the same problem on Air Canada in late November except booking online there generates no extra charge. This appears to be a bug in Mastercard's software which they could care lee about.

Booking by telephone at Delta with all the flight numbers,I was misbooked on one flight that cost an extra hundred dollars. The original flight was still available online immediately after I booked.

I will be changing credit cards and airlines soon.

Bob