Hi, I can fix the issue which I guess is because of some extra which is being sent in download manager plugin which breaks some cookie setting that is made by checkout plugin. Just a guess, if it is something different I will find it out, existing plugins debugging is my favorie type of task as it requires more thinking and less typing. :)
Please check my profile's feedbacks and reviews to see the proof of my experience and expertise in Wordpress. There is more than a hundred of excellent reviews and all of them received the highest ratings, all rated 5 out of 5 with 97% completion rate, that is really rare case when ALL employers are fully satisfied! Please compare to other freelancers ratings, their average usually is 4.9 or 4.8 and 60-80% completion rate meaning that they failed sometimes.
I have successfully completed hundreds of different WP projects starting from minor theme or content updates to huge and complex WP plugin development and dozens external API integration tasks. I have 11 years experience in Web Development, 5 years with Wordpress. My favourite technologies are PHP, MySQL, HTML5, CSS, JavaScript + Ajax + jQuery + libs.
Comparing to other freelancers who are spamming with their bids I am actually reading your project description and bid with precision only on projects I like. I am located in Latvia, North Europe and I work mostly with employers located in Europe and North America.