If your letter does not arrive one of the most likely causes is fake postage stamps. This is quite an issue these days. Royal Mail has a very helpful page on the subject of how to spot fakes, but the main message should be price ... if you are paying less than 95p for your first class stamps then they are most probably fake. If someone offered you a £5 note for £4 you would be suspicious, and the same should be true of stamps. Some are so obvious as to be laughable, with poor colour and bad printing but most are quite convincing to the casual glance. Genuine stamps have the words 'Royal Mail' in wavy lines printed in the background of the queen's image. Some fakes have this background printing but if you look closely it may be backwards, have poor printing or be in straight lines. 'They' have apparently already managed to fake the new barcoded stamps (that were supposed to prevent fraud), though I have not come across any yet. I had one customer who had bought her book of stamps from a local corner shop so be cautious if you buy them anywhere other than the post office. People online claiming to buy stamps in bulk cheaper are lying, you can only get cheaper postage if you have a business account (and this is metered post not using stamps). The sorting machines are pretty clever and can spot fakes and filter them out of the system to be processed by Revenue Protection. Underpaid items will be surcharged and delayed by at least a week.
Flowers by post seems like a lovely idea (not a fan myself, I like my flowers still attached to the plant) but, if you send someone flowers as a gift, tell them! Every week we return dead flowers that nobody has collected. Often the recipient is not expecting anything and does not pay attention to their P739 card. Sometimes people call in a week later, by which time the flowers are pretty shrivelled and unappealing. These nice flat boxes are designed to 'go through the letter box' ... but only certain kinds of letter boxes. Just give them a call and ask if the flowers arrived, or check your tracking number to see if they have been delivered. It feels like such a waste that so often nobody gets to enjoy them.
(Disclaimer: this A to Z is not official Royal Mail advice, except by coincidence.)
Stay safe. Be kind. Stop and smell the flowers.