Tawheed means not believing nor worshiping but only Allah (SWT). Shirk is believing or worshiping other than Allah with Allah (SWT). We believe that the Prophet and his Ahlul Bayt are the most humble servants of Allah.
Prostrating at the doorstep of the Prophet (SAWA) is in fact for thanking Allah for granting us the great bounty of visiting the shrine of the Prophet (SAWA) or his Ahlul Bayt (AS). Thanking Allah in the form of prostrating is a sincere worship to Allah and can never be Shirk. We never believe nor accept any claim suggesting worshiping the Prophet, his Ahlul Bayt or any one but only Allah (SWT). Respecting the Prophet and his Ahlul Bayt does not mean worshipping them at all. Respect is completely different from worshiping. Wahhabis mix wrongly between respect and worship. Even we prostrate at the doorstep of the holy shrines, we never do it as a sign of respect but we do it as an act of thanking Allah for His great bounties on us.