As noted by softech, you'll need to contact support. I can however attest to the fact that they probably do have a tracking number for the package, just it may take a bit for them to get back to you. You'll need to find the order number and provide...
You did sort of answer the question yourself though: Probably the biggest perk of the Public etc version of mobile is the ability to just drop the service or change it. Yes it's absolutely frustrating in instances like that 35$/20 gig thing last we...
That's the 20$ question the last week or so. For the most part the official word is that it's for new customers only. Some have had luck changing their plans with some tricks but then it becomes a question of whether the plan will stick or be honor...