how can i sum first day value of each id together?
Just a suggestion:

Let the length of the most transaction over all ID be $n$.

Prepare a list of size $n$, $A$, initialize all entries to $0$.

Now go through each ID, suppose a particular ID has $m$ transaction.  We add the value of the $i$-th transaction to the $i$-th entry of $A$.

The list $A$ would have your desired information. The $i$-entry stores the sum of values of the $i$-th day.

Alternatively, you can also introduce a new feature that encode which transaction  is it for each ID and then use groupby to sum it up again.