Cashback is credited for purchases of tickets only. Its amount is calculated on the cost of the ticket, excluding taxes and fees. Ticket price displayed in your LetyShops account is not corresponding with the actual one, but cashback is credited properly according to the stated rate.
The ticket price after transferring to Aviasales website from LetyShops can differ from the price you can see without such transfer. It is connected with the frequent changes of prices on the booking service.
Time for crediting cashback is counted from the flight date.