Thanks for the detailed report! I reproduced the offer and found a tutorial bug: the guide blocked access to the price breakdown after choosing a tactic.
The calculation is 7 (70% of base value 10) +4 Tech −4 Ritual +8 first-port bonus +3 Best angle = 18 credits. After the 1-credit service fee, you receive 17.
I’ve published 2.6.1: the first-sale breakdown opens automatically and stays accessible during the tactic and sale steps. Net proceeds and presentation costs are clearer too. Selecting a buyer is free; revealing a new combination uses one of three presentations, including a refusal. Revisiting an unchanged offer is free.