único/única is an adjective (that's why you can change it from o->a and add an plural "s")
somente is an adverb. and is always fixed, never any "s" in the end.
i e, when referring to a noun, use único. When referring to anything else, a verb, an adjective etc, use somente. In this text "only" refers to "sell", "we only sell".
Other examples: We only produce bananas = nós somente produzimos bananas. Referring to "produce".
They are only valid today = eles são válidos somente hoje // eles sao validos hoje somente (I dunno which one is better)
I am the only man here = Eu sou o único homem aqui (referring to man)
"We sell only blue bags" is general (while oddly being quite specific in potentially limiting the merchandise in the entire store to just blue bags).
"We sell the bag in blue only" (or only in blue) is specific to that bag which may be one of many styles they sell (along with dozens of other items such as clothing or office supplies) but, for this specific bag they happen to only have it in blue.
But that bag (purse) might come in a different color at a different store (or online), or it may be a limited edition in different colors which are hard to find (and this shop does not have it).
As usual, context can mean all the difference in the world to understanding why a sentence works or not.