Page Cycle Question / SW Architecture

I've design my app in a non-standard way, primarily because when I started I didn't know how the page cycle worked and so I tried to treat the whole thing like it was happening in Access.   Now I suspect I'm using the page cycle inefficiently (in fact I know I am) and I'm looking for general information to see what I can do about it.

The app uses one "Main Form" for user input data (other than admin stuff).  The Main Form gets its primary data inputs from either a callback string or a hidden field, both of which are loaded by javascript at the client.  

In either case, during PageLoad, this input string is used as parameter to a single function, call it "ProcessTransaction", that is an entry point to a large set of business process classes.  These classes do things like build context menus, format input transaction rows for the gridviews, edit input data, and update accepted transactions to the database.

On return from "ProcessTransaction" I run another function, in the page code behind, call it "BuildPage", that formats the page and does gridview databinds.   In the same code block there's also a default execute of "BuildPage" in case of a simple "Refresh" type submit that doesn't require ProcessTransaction.

PageLoad
   ProcessTransaction
   BuildPage
 or
   BuildPage (on a simple refresh -  BTW, it's a custom refresh button, not F5)

While I kept kind of jury rigging it all into place while I went through my ASP.NET learning curve, it now all works fine, and because the BuildPage executes the gridview databinds, the gridviews somehow automagically know NOT to rebind.

The problem is, which I discovered later, that when I use a regular ASP.NET button, with no client side processing, the software executes PageLoad first, and then executes the button click event.   These buttons typically do things that don't require "ProcessTransaction".  However, they might modify the page appearance or the data selection.  And so I then have to run "BuildPage" again.

PageLoad
   ProcessTransaction
   BuildPage
 or
   BuildPage (on a simple refresh)

Click Event
   (modify data selection)
   BuildPage

Since I can't detect the button event before PageLoad, I can't skip the ProcessTransaction block on the PageLoad pass.

So, I'm trying to figure out if there's someplace else to run ProcessTransaction besides PageLoad, that would cause it to execute AFTER the button click event.

Since this is "just" a performance issue at this point, I'm going to approach any change cautiously, and I recognize I'll have to make sure that all the other PageLoad actions are properly coordinated with ProcessTransaction.

The input I'm looking for from EE is: where I can put ProcessTransaction/BuildPage so that it will run AFTER the runat server buttonclicks...and any warnings that might be associated with that suggested location.

Any guidance on this would be appreciated.

Thanks!