In "Stamps" by Bethlyn Madison Webster, I believe the symbolism is found in the title and the several occurrences of the use of the word "stamp" throughout the poem.
Literally, the first stamps are "food stamps," which are given to people who don't make enough money to be able to eat properly. There is a certain "stigma" with some people about those who need food stamps, along with deep-seated embarrassment by those who use them. It is a sad commentary that the check-out clerk thinks she has the right to study the food on the belt in judgment, and even sadder that the speaker of the poem feels the need to apologize for needing the stamps.
I think she wants
to see how her tax dollars
are being wasted today...
The second use of stamps is the "rubber stamp" that the cashier uses to count the stamps presented. I can only imagine that the stamping makes noise, announcing with each stamping motion that the tickets are not just being counted, but voided—the sheer act delivered with emotions similar to anger, resentment and censorship.
The checker lays the coupons upside-down,
like a blackjack hand,
and pounds them with a rubber stamp.
However, figuratively, I find that the idea of "stamps" in this poem points to what we often term as a "stamp of approval" or "the Good Housekeeping" stamp (or seal), both of which express acceptance and merit. There is nothing about the cashier's attitude to show sympathy, compassion or approval. If living a life making only ten cents above minimum wage, with a child to support as well, isn't difficult enough in itself, it is conscionable that the woman at the register also feels it her right to also be critical and selfish in her nature.
The poem speaks to the audience to point out the hurtfulness of judging, the lack of compassion some people demonstrate, and the need to draw attention to another person's difficulties in life.
The stamp symbolizes the pain and embarrassment of needing to ask for help, the censorship delivered by the uncaring cashier, and society's sense that the situational details of someone's life somehow summarize the value of that person's life, which is totally inaccurate and extremely unfair.