not really sure here but along these lines IMO

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Note added at 56 mins (2021-08-31 02:19:58 GMT)
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or: overestimates

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Note added at 57 mins (2021-08-31 02:21:15 GMT)
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When costs are overestimated, the owner ends up paying more than necessary or may decide not to proceed with the project. Contractors can also face negative consequences if they overestimate project costs: they are likely to lose in competitive bidding or they may be regarded as scammers, hurting their reputation.

seems more likely than 'overpricing' here;

I think this is what they mean. See https://en.wikipedia.org/wiki/Cost_overrun .

rather than an extra charge and, pace Daryo, triggering a credit note from the customer to the wholesaler who might have 'volunteered' extra value by mistake e.g. by supplying two machines instead of one - not unknown vs. moins-value as a value markdown.

Plus value could also be a goodwill entry so excess value on a/cs.

Added value to a quotation is the typical expression to make sense of this sense of 'plus value'.

https://www.forumconstruire.com/construire/topic-75390.php

What'is it about? To determine whether a quotation's value can be increased, to which French law provides many responses, as detailed in the following attachment:

https://www.mesalertesetconseils.fr/2016-09/devis-etabli-et-...

According to the text submitted by Asker, the way to benefit from an added value is to reject any deal with no room for renegotiation.

The way to shun the inconvenience of any added value is cost-cutting.

Wikipedia proposes the following definition of 'plus value':

Littéralement, une plus-value désigne une augmentation de la valeur d'une entité qui n'a subi aucune transformation ou modification matérielle. Néanmoins, il est possible de rencontrer ce terme dans un contexte de modification de contrat d'une transaction.

Cas concret, lors de la signature d'une notice descriptive avec un constructeur immobilier, tout changement, modification ou ajout de votre part, à la suite de cette signature, peut entraîner une plus-value sur la construction.

Par exemple, l'ajout d'un appareillage électrique, telle qu'une prise de courant) fera l'objet d'une plus-value par rapport au contrat initial.

De la même façon, le choix d'un carrelage de sol plus grand ou de taille différente entraînera aussi une plus-value. À coût de marchandise équivalente, il est possible que le coût de main d'œuvre soit plus élevée.