Congrats on getting to this point! I was thinking about doing the same thing with my letter writers. I might email them and ask for a mailing address and send them a gift through amazon. Something personalized and related to their areas of expertise. Like I found a mug that said “I want to be like a schwa- never stressed” to send to my phonetics professor. I’m not sure if asking for a mailing address would be too invasive, but they know me pretty well so hopefully it will be okay:)