The price adjustment is due to EU forcing Ali etc to pay VAT for products below 150€. Above 150€ VAT is displayed in the offer, but will be deducted later ("price adjustment"). For the products above 150€ customs and VAT would have to be paid by customer.
With sellers shipping method the seller takes care of customs and VAT. Hence the higher price for the shipment
Thanks for the reply, this is the answer I received on an Italian forum that cleared up all my doubts: "by choosing shipping with prepaid duties, the Aliexpress price is the final price, you will not pay anything else. The package does not pass through Italian customs, but is cleared abroad (Belgium, Holland or even in other countries) and delivered to the courier who transports it to the final destination. The courier who transports from China to Europe takes care of customs clearance.
If the price on Aliexpress is higher than 150 euros (in reality it reaches about 170/180 euros) and the shipping is the standard Aliexpress, the package is cleared in Italy and, in this case, VAT is applied together with duties and postal charges."
At this point I have no choice but to take advantage of the Aliexpress discounts and order the frame.