It is not a big deal at all. But, as said before DO NOT RELY ON ADCUS.
Get the phone numbers and call. Sure, it might be international rates, but just last week I witnessed Customs STRIP a Citationjet at Tamiami. And, then put big old stickers on it. (siezed)
No, they didn't find anything, just showed up without notification.
And, be careful on the hours. some custom stops here are weird hours of operations. 11-7, 10-6 etc. So, just do a little bit of homework, and you will be fine.
Also, even though you physically called on the phone, an update through Miami FSS as you travel across never hurts.
And, if you decide to go VFR back, call Miami Radio and get a Custom's Transponder Code as soon as you lift off.
Enjoy the view
Hung