Tawassul is a matter which has been misunderstood by many Muslims now a days under the Wahabi propaganda which wrongly claimed it as a type of worship to other than Allah. Wahabi sect claims following Tawheed but their books prove that they do not understand the real meaning of Tawheed in Quran and authentic Hadeeth. That is why they accuse other Muslims to be Mushrik just because they don't follow the Wahabi misunderstanding of Tawheed.
'Tawassul is seeking the help of the most humble servants of Allah to get our needs. We believe that they (The Prophet and Ahlul Bayt (AS) are Servants of Allah and only do what Allah wants. There is no question at all of Shirk because Shirk means to believe that there is a partner or helper etc to Allah.
Addressing the Prophet is absolutely right as he himself taught Muslims to address him in Tawassul as it has been narrated in Sunni and Shia books in the famous Hadeeth of the blind حديث الأعمى which is narrated by Othman Ibn Hunaif that a blind person requested the Prophet to help him get back his eye sight. The Prophet told him to tolerate but he said : I can not tolerate. The Prophet advised him to perform Wudhu and pray two Rak'ats of Salah and say in the last Sajda: O Allah, I seek from You by Your Prophet, the Prophet of mercy. O Muhammad; I am seeking from Allah through you to grant me my need.
'The blind did what the Prophet taught him and got back his eye sight. The Hadeeth is narrated in al-Tareekh al-Kabeer by al-Bukhari, and Tirmithi and Ibn Maajah and Nasaa'ee and al-Bayhaqi and Abu Na'eem in Ma'rufat al- Sahabah, and al-Tabaraani and Ibn Hajar al-Asqalani and al-Soyooti and many other well known Sunni scholars.
'We can address the Prophet (SAWA) and Ahlul Bayt (AS) directly knowing that they only do what Allah wants, as well as addressing Allah directly with the Waseela Bihaqqi of Ahlul Bayt (AS). Both are good.