In your context, it shoud be "allocated to".
Consider the following explanatory sentences:
Three computers have been allocated to me.
(Meaning: I was given three computers)
Three computers have been allocated for developmental purposes.
(The speaker of this sentence is telling us the purpose for which the computers have been allocated)
Another way:
------------
Your sentence is in the passive. Let us trun it into the active.
The order of 1200pcs has been allocated to ABC customer (Passive).
They have allocated ABC customer the order of 1200 pcs. (Active)
Here we have two objects:
ABC customer= indirect object
the order of 1200 pcs: direct object.
When turning the sentence into the passive, as a rule, the indirect is preceeded by "to" and not "for".