Increasing load cost will affect shipping costs, but just nudging them up to 10 shouldn't be too large a change. Alternatively just leave them as they are and just don't use the ability to transport them by air.
The only Allied air-transportable tanks were the M-22 Locust and the Tetrarch, both could be carried by the British Hamilcar glider, while the US could only transport the Locust by carrying the turret inside a C-54 and the hull underneath the fuselage - obviously this wasn't a practical way to transport them into combat. Use of both was very limited. The UK formed 3 independent Tetrarch squadrons - A, B and C Special Services Squadrons, initially for amphibious use, used C Sqn for the amphibious landings in Madagascar, after which it remained in-theatre for a while before being withdrawn to the UK, where subsequently it was used to form an armoured squadron for 6th Airborne Division which landed at D-Day, and which then was expanded to form an armoured regiment that took part in the Rhine Crossing. The Tetrarchs were replaced by Locusts before the Rhine operation, but then that decision was reversed and ultimately a mixture of Locusts, close-support Tetrarchs and Tetrarchs with the 2-Pdr fitted with the Littlejohn squeezebore adaptor were used. US usage was even more limited, with 1 company and 1 battalion formed on Locust and never deployed operationally.