If you agree to let them do it, I assume that they can raise the contribution by whatever amount you agree to.  It would clearly be a problem if they raised the amount without asking permission.   Unilaterally raising the amount without notification would most likely be an illegal breach of your current contract. It would also be a problem if they made it a recurring contribution without telling you or without making it clear that it was to be a recurring gift when you signed up. The Trump Presidential campaign and the Republican National Committee refunded more than 530,000 contributions totaling about $64.3 million in 2020 to donors who complained that on-line donation forms were inconspicuously pre-checked to be recurring donations when the donors had no intention of doing so, according to the New York Times.  The article did not claim that the pre-check boxes were illegal, but critics called the practice unfair and unethical.

You have the opportunity to opt out and should definitely do so if you don’t want to give more.  You might even decide that you will want to opt out of making any additional donations going forward.  The number of people opting out of a situation is almost always less than those who have to opt in, and I assume that your charity intentionally made it an opt-out choice and not an opt-in one.  

What might be legal may nevertheless be so distasteful that it impugns the integrity of the organization overall.  I wouldn’t recommend this trick to any charity that relies on the trust of its donors to raise funds.