Pamela's virtue in her tale is said to be rewarded, and certainly in a 18th century view it would seem as such. With a modern sensibility, however, it seems more likely that she suffers of Stockholm syndrome and was duped into her new higher station.
Pamela had been released from Mr. B. When he sent a letter after her asking for her to return to him by her choice. Although this may seem like a kind change of character, that he was transformed upon his reflecting on Pamela's good nature as he claims, elements in how he and the others among the higher classes treat her that do not provide any sense of rewarded behaviour or comfort. Granted her sudden leaping up in rank would have been difficult to assess and treatment of her would have been an element of concern, other than her rise in rank a modern reader would not see her as rewarded; a better reward would perhaps have been her choosing to be free and helping raise her family's condition.
Within an 18th century context, however, her sudden and drastic rise in rank would have been more than enough reward for her virtuous attitude, and it is not difficult to see why some might have seen it as exuberant. Class was of the utmost importance at the time, and as such the numerous ranks that Pamela jumps over in her marrying Mr. B. is substantial one terms of reward. Your worth in society was heavily reliant on your class, and raising to such nights from such humble a position would be rare at best.
Pamela's uncertainty and problems at the end of the novel stem from her habitual virtuous and humble attitude that causes her to be so afraid to run in the beginning. Her attitudes at the end of the novel make sense in considering her previous station and her virtuous personality. Her desire to remain proper but not knowing what being proper meant in her new role leads to her uncertainty in the later half of the novel, and explains why her happy ending may not be as happy as one may come to expect in a modern view, while also being a perfectly acceptable one for the 18th century.