Verizon's website is notorious for being able to handle only one transaction at a time.  So changing your plan AND making a purchase, as you  discovered, doesn't work well.  Calling CS as the rep suggested will allow you to do it all at once.

To do it all on your own, you'd need to change your plan, establish that it is in place, probably wait a billing cycle, THEN try the purchase.  I'd call in and purchase and request the plan change.