Beware, the percentage quoted in various texts and on bottles is not always calculated the same way.
There are two factors that make a difference. One is percentage by weight or volume. If a percentage is quoted in most research papers it is a percentage by weight, sometimes abbreviated w/w. There is an older (obsolete) way of percentage by volume where you take x grams of substance and make it up to 100 ml. Percentage by volume is what is on the Endolapi SRL (Italian) labels. It makes only a small difference for oxalic acid, a larger difference for sugar. The second factor is whether the original weight was the hydrate version. When crystals form there is often some water in the crystal structure, that should be included in the calculation but it's not on the Italian labels and it makes a larger difference.
Endolapi SRL (the Italian company) produce oxalic in three concentrations, those are the bottles with "l'Enologia e l'Apicultura" at the top (Winemaking and Beekeeping that the company supplies products for). The label concentration is 6% (yellow), 4.5% ('New Zealand formula' pale blue) and 3% (white). How they calculate the percentage is clearly stated on the bottles but it's not the percentage as it is used in scientific papers, for example
http://www.apimondia.com/apiacta/articles/2003/nanetti.pdf The bottles are also clearly labelled as in 30% sugar solution, that's below the usually recommended sugar concentration but it may keep longer.
The 4.5% Oxalic on the label is actually close to the 3.2% usually recommend for the UK when calculated by weight of the anhydrous oxalic acid. The 6% labelled is actually about 4.2% by weight and the stronger concentration commonly used in Italy. The 3% labelled is around 2.1% by weight and used by some in Scandinavia.
Why do the Italians label it like that? Why do P4aynes and others sell the higher concentrations and not make the distinction clear? You'd have to ask them. Confusing? Yes, and it's why for most scientific purposes concentrations are quoted in molarity. Harder to work out initially, but unambiguous.