Solutions
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Heenrique's post in (Resolvido)Erro com Shop System was marked as the answerResolvido.
Apenas troquei:
#=================== CODE ======================#
include_once('./engine/load.functions.forum.php');
#===================================================#
por:
#=================== CODE ======================#
include_once('./engine/load.functions.shop.php');
#===================================================#
e deu certo, mas muito obrigado pela ajuda Absolute!