The United States's No. 4 semipostal stamp: Save Vanishing Species (Scott No. B4)  was originally sold nationawide
from September 2011 through December 31, 2013. The surtax was donated for multiple national species conservation
programs.

But only 25% of stamps were sold (25 million out of 100 million stamps printed)

The USPS then decided to place the stamps back to sale until December 31, 2018.

adapted from Linn's Stamp News
Published 10/13/2014

When the Save Vanishing Species semipostal tamp (Scott B4) was issued Sept. 20, 2011, the postage value was 44¢ and the surtax was 11¢.

When the stamp was taken off sale (dec.31, 2013), it was selling for 55¢, which included the postage value of 46¢ and a surtax of 9¢ at that time. The current first class mail postage rate is 49¢.

A total of 24,989,000 stamps had been sold as of November, 2013. Based on its sales, the stamp has earned approximately $2.52 million for the funds.

Had it not been approved by the congress, the unsold stamps ¡X approximately 75 million of them would have headed to the shredder and recycled.

semipostal stamps are originally designated for fund raising.
I just wondered why the USPS set a deadline for the sale

I believe you've asked questions related to semi-postal stamps in another thread.

The USPS does not operate like the post office in Taiwan. They do not have much "power" per se on selling semi-postal stamps. In short, the USPS was forced to sell such kind of stamps by the congress. They don¡¦t want to keep losing money selling them due to the complexity. They need to do tons of extra work to keep track on the money and inventory. Remember, it is an assignment given by the congress, and to them, it¡¦s nothing but a big money pit. Then you need to report to the congress and explain why if it does not sell well, not to mention two reports to the president. USPS would love to see those stamps expired and never have to deal with them again.


Below are some of the facts¡K¡K¡K¡K¡K¡K¡K¡K

Europe has a long tradition of using semipostal  stamps to raise funds for worthy causes. Some of the causes supported by European semipostals include child health, literacy programs, national sports development, and philately (stamp collecting).

In the United States, however, semipostals are a recent innovation (not until 1997). USPS has long opposed their issuance. While commemorative stamps have from time to time been issued to raise awareness of social or health problems in the nation, USPS was reluctant to get into the fund-raising business. USPS argued that there was a strong tradition of private philanthropy in this country, and ¡§due to the vast number of worthy fund-raising organizations in existence, it would be difficult to single out specific ones to receive [semipostal] revenue.¡¨ USPS also warned that the administrative costs involved in accounting for sales would tend to outweigh the revenues derived from the surcharge.
Philatelic groups also opposed semipostals. They generally thought that USPS was
issuing too many commemorative stamps, with a broader clientele in mind than that of the stamp collector. Semipostals were a departure from the tradition that stamps are for postage. Collectors regarded the semipostal surcharge as a tax on their hobby, and pointed out that unlike other citizens who might be unsympathetic to the cause being supported, they still had to buy the stamp or their collections would be incomplete.

The Breast Cancer Research Stamp
Despite USPS opposition, Congress authorized a semipostal stamp for the benefit of breast cancer research in 1997. Inside such Public Law, the congress dictates
1.    Issuing date.
2.    Termination date.
3.    % goes to charity.
4.    Cost control ¡V designing, printing, distribution, sale, overhead.
5.    Submit interim and final report to president of the United States during the program.

The termination date is the time which congress estimates that by that time USPS will have achieved the fund raising target amount.

Hope this would help.