I commute to Detroit for work everyday on H1B (AVR). My Extension was filed today. Following are couple of points my attorney highlighted:
1) If you are not in US when applying for H1B Extension, your H1B will be adjudicated for consular processing (which requires H1B Visa to enter USA)
2) On the day when your H1B Extension package is received by USCIS, your I-94 gets adjusted (assuming you are in US on the day of receipt) for 240 days allowing you to work while awaiting decision.
3) After the case is received and receipt# is generated, you are good to continue commute (US/Canada).
I am not sure if this is helpful, but thought of sharing it.