Except that it was made clear that drop tanks (or rather, the capacitors that enabled their use) were a recent invention in the OTU. So while it may change shipping costs in the future, currently (Classic Era) there are no contradictions involved).The drop tank idea date back to The Gazelle Close Escort. It was however provided with a crude rule and likely to upset the Economic of Free Trader play balance. So all kind of home rules were set-up (if you trust the talk here). Those that liked the idea made the tank reusable and easy to fit back, those that did not like it insisted that they had to be custom design, fitted in yards, (time and money).... By and large, there was no rules to play balance the advantage and that would upset the OTU.
Hans