Instead of saying that you over-ordered you say that you bought too much, example:
Comprei comida demais no MacDonalds (I bought too much food at MacDonalds)
1. Definition and usage of to order (encomendar) in portuguese
The problem is that the word "encomenda" usually implies a lapse of time between the buy and the usage/consumption of an item. See the definitions at priberam
en·co·men·dar - Conjugar verbo transitivo
Encarregar de fazer ou dizer (alguma coisa).
Proceder à encomendação de.
Nomear (pároco encomendado). verbo pronominal
Entregar-se (à protecção de alguém).
In fact you may take that a step further and say that a delivery is expected, or that the person needs to pick it up.
This happens more frequently over the phone, or internet. It can also happen at the shop when the item is not available, or takes time to prepare, consider for example a pizza.
Let's consider the following examples.
Encomendei leitão para o jantar. (I ordered piglet for dinner)
Encomendei frango, mas vou buscá-lo á loja. (I ordered chicken, but I am picking it up at the shop)
Quero encomendar uma pizza para o jantar. (I want to order a pizza for the dinner)
2. An item that you are buying right now is not a "encomenda" it's a "compra"
In the situations we purchase an item and have it handed out in that moment we would prefer the usage of compra(r).
Eu comprei um relógio. (I bought a watch)
So over-ordering food in MacDonalds would translate as follows:
Comprei demasiada comida no MacDonalds (I bought too much food in MacDonalds)
3. You can use the same concept with encomenda.
Encomendei demasiado leitão. (I over-ordered piglet)
Notice however, that act of over-ordering happened in the past in such cenario. It couldn't really happen in the future, unless you are over-ordering on purpose.
Vou encomendar muito leitão (I am going to over-order piglet)
4. Over-ordering in financial context
In a financial context, the usage of "over-ordering" could be translated as "encomenda excessiva", as noted in the title. The following sentence
We reduced over-ordering by 30% compared to last year
Could be translated as follows
Reduzimos as encomendas excessivas por 30%, em comparação com o ano anterior