Apple told Memory Map to effectively remove the online map shop from the app some months ago. One way to buy is to order the cd & when it arrives transfer the relevant files to your iPad using iTunes.
I get these emails/newsletters
occasionally with links & special instructions for iOS users which seem to suggest that if you buy a chart through your browser it will appear in the list of downloadable maps.
Worth a £20 punt perhaps. If it doesn't work complain to MM & get them to send you a cd or email the files; worst case, claim your money back from your credit card.
As a pc & Android person I have no practical knowledge of iOS & in the absence of any clear instructions from MM I would be interested to know if the buy in browser, download in app method works.