and this is where you really shoot yourself in the foot
he did accept the return, but just not for 230€, WHY?
because you didn't pay 230€ for the Maths!!!!
you paid 400€ (and he received 397€) for a Maths and an Optomix and postage and packing and paypal fees
take off the postage and packaging, 12€ - he gets 385€, take off the Optomix at 180€, he gets 205€ for the Maths - seems reasonable - I would offer you the same deal - you only paid 205€ for the Maths, not 230€
go try this at a shop: buy a two for one offer on something and then try to get a refund on one of the items - I'm sure they will gladly take it back off you - but I would be really surprised if they give you any money
same with shops where they offer 2nd item 1/2 price - the price is the most expensive plus half the price of the cheapest - not the cheapest plus half the price of the more expensive item - but if you then take the more expensive item back they will want you to pay full price for the cheaper item
It really does not look like pibou is at fault here at all
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities