how much would the shipping rates be? I did find one post on another board where some people shipped home some stuff through La Poste with a flat rate box (40 euros). Can la poste be trusted?

Take the proof of purchase of that laptop with you. Ideally if it has a serial number. Then, declare this upon arrival to Europe as your own property and claim to take it back to the US. This may require filling out a customs declaration form. The customs clerks at the point of entry should be helpful. When you return to US, before packing the laptop to be sent back, get that original Customs Declaration you were filling in when entering Europe, let the European customs stamp it again (as outgoing item) and put that form inside the packing of your laptop along with American proof of purchase. That way, when the U.S. customs open the package, they will know that this laptop was purchased in the U.S., brought to Europe and sent back, and that the VAT was paid for it in the U.S., thus there is no need to impose an import duty on it. Serial number should be the same on the laptop, on the proof of purchase, and on the Customs Declaration from the EU customs office.