Buying a lobster from a store and setting it free could do more harm than good. This is because you might increase demand to the extent that the supply goes up by more than 1, and it wastes your time.
On a small scale:
| Do buy a lobster & save it | Don't buy a lobster & not save it |
|---|---|
| 1 Lobster saved per person | 1 Lobster not saved per person |
| ∼1 Lobster captured due to increased demand per person | ∼1 Lobster not captured due to increased demand per person |
| Lobster store gets money | Lobster store doesn't get money |
| You and other participants lose some time from your days | You and other participants don't lose some time from your days |
Also, doing it on a large scale adds another component. It might encourage people to recognize the rights of non-human animals, but it might also drive demand and increase the popularity of lobster stores, meaning it might do more harm than good.
On a large scale:
| Do buy a lobster & save it | Don't buy a lobster & not save it |
|---|---|
| 1 Lobster saved per person | 1 Lobster not saved per person |
| ∼1 Lobster captured due to increased demand per person | ∼1 Lobster not captured due to increased demand per person |
| Lobster store gets money | Lobster store doesn't get money |
| You and other participants lose some time from your days | You and other participants don't lose some time from your days |
| It might produce a higher view of non-human animals in the public eye | Won't produce a higher view of non-human animals in the public eye |
| It might build publicity for lobster stores, increasing demand. | It won't build publicity for lobster stores, increasing demand. |
Maybe edit your post? I don't know, it's really up to you.