The tension in Webster's poem "Stamps" is between the shopper who looks back at the woman and her husband and child in line at the grocery store. The shopper inspects what the other family is buying because they have food stamps, public aid for which the shopper's salary deductions are made. Critically, then the shopper eyes the chocolate that the family is buying since candy is not an essential food. Obviously, the shopper feels insulted that her "hard-earned money" should pay for candy. On the other hand, the woman is humiliated that the shopper should feel the right to inspect what they buy because they do try to support themselves, but the salary falls short of subsistence level. In further degradation, the family has their food coupons turned over and rubber stamped, thus making the tender less than money and clearly nothing that they have earned.
So, the theme of this poem can be stated in terms of the "stamping" of people, labeling them all as shiftless, who must use government aid. The woman who is so labeled feels shame and humiliation; she wants the shopper to know that they do work, but what they make is not enough to support a family.