Online shop, uplifted about sign of gratitude, to repay in kind (9)
I believe the answer is:
retaliate
'repay in kind' is the definition.
(to pay back in the same way)
'online shop uplifted about sign of gratitude' is the wordplay.
'online shop' becomes 'e-tailer' (online retailer).
'uplifted' says the letters should be written backwards.
'about' indicates putting letters inside.
'sign of gratitude' becomes 'ta' ('ta' can mean thanks).
'etailer' back-to-front is 'reliate'.
'reliate' enclosing 'ta' is 'RETALIATE'.
'to' is the link.
(Other definitions for retaliate that I've seen before include "Exact retribution" , "Return like for like" , "Act in revenge" , "Get one's own back" , "Strike back, avenge" .)