Reload Page Data Based On Dynamic DropdownList Selected Value

I have a bit of a tricky scenario. It consists of some partial problems for which I have individually found solutions, but I couldn't find anything that makes it all work together smoothly.

-Upon selecting an item in a datagrid, a new page loads with the details regarding that entity.
-The entity represents a product that can have any amount of variants (such as a t-shirt's color or size)
-I dynamically add dropdownlists to this detail-page that represent these variants (there can be any amount of variants, and each variant can have any amount of different values, each value or combination or values from different variants representing a unique product entity)
-Upon selecting a different variant in a dropdown, a delegate is called that will change the session variable to the productId associated with the selected variant and a postback is fired that will reload the page with new data based on this productId

The problem: the postback is fired BEFORE the dropdownlist's (OnSelectedIndexChanged) delegate is called. Since this delegate defines the productId in the session, the new to-be-used productId isn't known at the time of postback.

Result: loading the correct data in the page is always delayed by one postback. Each time I change the selected index of any of the dropdowns, the page is loaded with data based on the session variable that the PREVIOUS OnSelectedIndexChanged delegate of the dropdown set.

I have a LoadData() method that sets the values of all textboxes, fields, labels etc. based on the productId stored in the current session. This method is called on Page_Load.
Private Property CurrentProduct As Product
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
LoadData()
[Code] .....

It seems that I haven't been completely clear on what does and doesn't apply to this scenario regarding Page Lifecycle.

1) My dropdownlists are added dynamically, which works fine on the first page_load.
2) The content of the dropdownlists is defined by what entity's details i'm loading on the page.
3) What entity's details I'm loading on the page is at first(not-postback) defined by the product I selected in a gridview in the previous page.
4) At postback, this entity is based on a sessionvariable containing the id of the entity.
5) This session variable is defined by what item in the dropdownlist I select.
6) This dropdownlist dissapears after postback since it's not defined on the page, but dynamically. So is the event.

So my guess would go towards some clientside script that stored the selected value before postback, but then I would be clueless on how to get that variable to the server since the logic that retrieves the entity is in the business logic layer which is far away from this page..