[aerohead]

I looked at pages of aftermarket add-on 'aerodynamic' goodies for the Megane II GT online.
There were no drag benefit claims offered for any of, what I'm going to call 'chin spoilers' listed.
It appears that these products are 'ornamental', 'decorative,' with no attribution to enhanced aerodynamic performance.
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Your car is reported at Cd 0.35.
The 1989 Renault Megane concept car reported to be Cd 0.21 by Renault. It had no 'airdam' of any kind.
The 1987 Renault Vesta-II concept was reported by Renault to be Cd 0.186. It had no front airdam.
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None of the really low-drag concept cars, century-old low-drag research models, or land speed record cars had 'airdams.'
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'Airdams', while they 'can' be associated with drag reduction, do not necessarily appear to be a prerequisite 'for' drag reduction, if the 'nose' of the car has already directed the leading edge flow where it is deemed best to go.
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Published airdam test results show quite a spread in performance:
* The 1983 GM-Pontiac Firebird Trans Am airdam, with wrap-around sides contributed a Cd 0.036 drag reduction.
* My 1984 Honda Civic CRX modified nose over-fascia airdam indicated a delta Cd 0.0157 drag reduction, from it's top speed.
* The 2013 GM-Chevrolet Malibu's drag reduction due to it's airdam was published at Cd 0.005.
* The 2015 GM- Buick Excelle GT airdam was credited at Cd 0.0064.
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I haven't been in Poland since 1997, and I don't know what the laws are like now, with respect to owner modifications.
Personally, I'd be hard-pressed to drop up to, 153-Euros ( like I've seen for some airdam extensions ) without some guaranteed level of performance.
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If you aren't running the risk of breaking some law, perhaps to could cobble together a cardboard and duct tape facsimile of an add-on chin spoiler, and see if it produces a 'visible' result at the fuel pump.